GIFT  OF 
Pacific   Coast 

.Trn  i  r»n  -.<  1     n  f* 


BIOLOGY  U&RAR1 


BOOKS   BY 
R.  BURTON-OPITZ,  M.  D. 


Text-Book  of  Physiology 

Octayo  of  1185  pages,  with  538 
illustrations,  63  in  colors. 

Cloth,  $8.00  net. 


Laboratory  Physiology 

Octavo  of  238  pages,  illustrated. 
Cloth,  $4.00  net. 


Elementary  Physiology 
12mo  of  411  pages,  illustrated. 


AN  ELEMENTARY  MANUAL 


PHYSIOLOGY 


FOR  COLLEGES,  SCHOOLS  OF  NURS- 
ING, OF  PHYSICAL  EDUCATION, 
AND  OF  THE  PRACTICAL  ARTS 


BY 

RUSSELL   BURTON-OPITZ 

S.M.,  k.D.,  PH.D. 

Associate  Professor  of  Physiology,  Columbia  University,  and 

Professorial  Lecturer  in  Physiology  in  Teachers' 

College  of  Columbia  University 


ILLUSTRATED 


PHILADELPHIA  AND  LONDON 

W.  B.  SAUNDERS  COMPANY 
1922 


BIOLOCV 

LIBRARY 

D 


GIFT  PACIFIC  COAST   JOURNAL 
OF    NURoINQ 


Copyright,  1922,  by  W.  B.  Saunders  Company 


MADE     IN      U.     S.     A. 


PREFACE 


THE  teachers  of  physiology  in  our  Medical  Schools  are 
greatly  handicapped  at  the  present  time  by  the  fact  that  the 
material  which  must  necessarily  be  presented  to  the  students 
in  preparation  for  their  clinical  years,  is  so  complex  that  it 
can  scarcely  be  dealt  with  in  detail  in  the  number  of  hours 
of  teaching  ordinarily  allotted  to  this  science.  It  would 
indeed  be  very  helpful  if  the  matriculates  in  medicine  could 
also  present  certain  credits  in  elementary  physiology,  in 
addition  to  those  obtained  in  biology,  physics  and  chemis- 
try. The  only  objection  that  might  be  raised  against  the 
establishment  of  more  extensive  courses  in  elementary 
physiology  in  colleges  is  that  this  change  would  tend  to 
cause  these  institutions  to  lose  their  identity  and  purpose 
even  more  than  they  have  at  the  present  time.  Obviously, 
a  college  is  intended  to  disseminate  general  knowledge  and 
not  knowledge  of  a  special  or  professional  type.  Thus,  a 
college  which  permits  its  matriculates  to  crowd  diverse 
courses  in  the  fundamental  sciences  into  the  first  two  years 
of  its  curriculum  and  then  passes  these  young  men  and 
women  on  to  the  medical  schools,  fails  utterly  in  its  purpose 
as  a  means  of  acquiring  general  culture. 

It  is  evident,  however,  that  the  mission  of  physiology  is 
much  greater  than  that  of  serving  as  an  essential  link  in  the 
chain  of  medical  subjects,  because  it  also  possesses  an 
eminently  practical  value  as  a  general  study.  This  fact 
is  recognized  more  and  more  from  year  to  year.  Thus, 
many  States  now  require  elementary  courses  in  physiology 
in  preparation  for  licenture  in  teaching  in  secondary  schools 
and  high  schools.  These  raised  requirements  have  greatly 
aided  in  destroying  the  erroneous  conception  of  the  school- 
boy that  physiology  is  essentially  a  discourse  upon  the 
evil  consequences  of  smoking  tobacco  and  drinking  alcoholic 
beverages.  Since  physiology  seems  to  have  been  presented 

743521 


12  PREFACE 

chiefly  from  this  viewpoint,  it  cannot  surprise  us  to  find  that 
this  subject  has  always  been  distinctly  unpopular  with  the 
pupils  of  the  secondary  schools.  As  stated  above,  the 
better  training  of  these  teachers  will  soon  change  this  atti- 
tude of  the  pupils,  because  physiology,  if  properly  presented, 
cannot  fail  to  arouse  their  undivided  interest. 

Physiology  is  destined  to  fulfill  an  even  more  important 
mission  in  institutions  for  the  training  of  nurses  and  dieti- 
tians. To  this  group  of  young  men  and  women  are  to  be 
added  the  constantly  increasing  numbers  of  students  of 
physical  education.  Clearly,  all  these  men  and  women 
should  be  thoroughly  familiar  with  the  structure  and  func- 
tions of  the  human  body  as  compiled  from  data  pertaining 
to  living  matter  in  general.  It  is  obvious,  however,  that  the 
material  which  may  justly  be  presented  to  them,  must  be 
more  elementary  in  its  character  than  that  offered  to  the 
students  of  medicine. 

In  order  to  meet  the  demands  at  this  University,  I 
established  some  years  ago  certain  courses  in  elementary 
physiology  which  are  now  attended  by  more  than  three 
hundred  students  during  each  academic  year.  These 
courses  consist  of  one  to  two  hours  of  lecture  and  two  hours 
of  practical  work  each  week,  or  of  about  one  hundred  to  one 
hundred  and  twenty  hours  in  all.  The  publication  of  this 
book  has  been  stimulated  by  my  desire  to  supply  these 
students  with  a  text  presenting  the  subject-matter  of 
physiology  in  as  simple  and  logical  a  manner  as  possible. 
If  I  have  succeeded  in  this,  I  hope  that  these  pages  will  also 
be  favorably  received  elsewhere,  and  aid  materially  in  the 
dissemination  of  physiological  knowledge  among  those  men 
and  women  who  are  not  directly  concerned  with  medicine 
but  are  nevertheless  entitled  to  the  benefits  that  are  surely 
to  be  derived  from  this  study, 

R.  BURTON-OPITZ. 

COLUMBIA  UNIVERSITY, 

NEW  YORK  CITY, 

February,  1922. 


CONTENTS 


PART  I 

THE  PHYSIOLOGY  OF  MUSCLE  AND 
NERVE 

SECTION  I 

GENERAL  PHYSIOLOGY 
CHAPTER  I 

PAGE 

LIVING  MATTER 17 

CHAPTER  II 
GENERAL  PHENOMENA  OF  LIFE 30 

CHAPTER  III 
GENERAL  CONDITIONS  OF  LIFE 37 

SECTION  II 
MUSCLE  AND  NERVE 

CHAPTER  IV 
MOTION 46 

CHAPTER  V 
THE  STRUCTURE  AND  GENERAL  BEHAVIOR  OF  MUSCLE  TISSUE  .  .  59 

CHAPTER  VI 

THE  MANNER  OF  CONTRACTION  OF  MUSCLE 67 

CHAPTER  VII 

ANALYSIS  OF  MUSCULAR  CONTRACTION 76 

13 


14  CONTENTS 

CHAPTER  VIII 

PAGE 
CHEMISTRY  OF  MUSCLE 86 

CHAPTER  IX 
THE  NERVE  IMPULSE  AND  REFLEX  ACTION  .  93 


PART  II 

THE  CIRCULATION  OF  THE  BLOOD  AND 
LYMPH 

CHAPTER  X 
THE  LYMPH.    .    .....    .   , 107 

CHAPTER  XI 
THE  BLOOD. ....../..   115 

CHAPTER  XII 
THE  GENERAL  ARRANGEMENT  OF  THE  CIRCULATORY  SYSTEM  .   .   .   127 

CHAPTER  XIII 
THE  HEART  OF  THE  MAMMALS 136 

CHAPTER  XIV 
THE  FLOW  OF  THE  BLOOD 152 

CHAPTER  XV 
BLOOD-PRESSURE  AND  RELATED  PHENOMENA 160 

CHAPTER  XVI 
THE  NERVOUS  CONTROL  OF  THE  HEART  AND  BLOOD-VESSELS.   .   .   171 

PART  III 
RESPIRATION 

CHAPTER  XVII 
THE  ELEMENTARY  LUNG 181 

CHAPTER  XVIII 
THE  MECHANICS  OF  RESPIRATION  .  .    188 


CONTENTS  15 

CHAPTER  XIX 

PAGE 
THE  CHEMISTRY  OF  RESPIRATION 200 

CHAPTER  XX 
THE  NERVOUS  REGULATION  OF  RESPIRATION 210 

PART  IV 
NUTRITION 

CHAPTER  XXI 
SECRETION 216 

CHAPTER  XXII 

SALIVARY  DIGESTION 223 

CHAPTER  XXIII 
GASTRIC  DIGESTION 233 

CHAPTER  XXIV 
INTESTINAL  DIGESTION 245 

CHAPTER  XXV 

THE  PROGRESS  OF  THE  FOOD  THROUGH  THE  INTESTINES-J-ABSORP- 
TION 255 

CHAPTER  XXVI 
METABOLISM   ....    ._-.,:•. 265 

CHAPTER  XXVII 
THE  METABOLIC  REQUIREMENTS  OF  THE  BODI — ANIMAL  HEAT.   .  272 

CHAPTER  XXVIII 

EXCRETION 284 

CHAPTER  XXIX 

THE  INTERNAL  SECRETIONS 295 

PARTY 
THE  NERVOUS  SYSTEM 

CHAPTER  XXX 

THE  FUNCTIONAL  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM  .    .    .   304 

CHAPTER  XXXI 
THE  SPINAL  REFLEX  ANIMAL.  .   311 


16  CONTENTS 

CHAPTER  XXXII 

PAGE 

THE  BRAIN 320 

CHAPTER  XXXIII 
THE  CEREBRUM.    . 327 

CHAPTER  XXXIV 
THE  LOCALIZATION  OF  FUNCTION  IN  THE  CEREBRUM 333 

CHAPTER  XXXV 
THE  CEREBELLUM  AND  MEDULLA  OBLONGATA 341 

PART  VI 
THE  SENSE  ORGANS 

CHAPTER  XXXVI 

THE  CUTANEOUS  SENSATIONS.     TASTE  AND  SMELL 352 

CHAPTER  XXXVII 
THE  SENSES  OF  HEARING  AND  EQUILIBRIUM .  362 

CHAPTER  XXXVIII 
THE  SENSE  OF  SIGHT 375 

CHAPTER  XXXIX 
THE  COURSE  OF  THE  RAYS  OF  LIGHT  THROUGH  THE  EYE.    .    .    .381 

CHAPTER  XL 
THE  STIMULATION  OF  THE  RETINA  BY  THE  RAYS  OF  LIGHT  .    .    .   387 


INDEX   .  395 


PART  I 

THE  PHYSIOLOGY  OF  MUSCLE 
AND  NERVE 


SECTION  1 
GENERAL  PHYSIOLOGY 


CHAPTER  I 
LIVING  MATTER 

Definition  and  Scope  of  Physiology. — Science  is  accurate 
knowledge  acquired  by  means  of  exact  observation  and  cor- 
rect thinking.  This  definition  suggests  that  scientific 
knowledge  is  gained  by  an  analytical  study  of  matter  as  it 
exists  in  nature.  The  universe  is  composed  of  different 
materials,  such  as  earth,  air,  and  water,  which  are  collec- 
tively designated  as  matter.  But,  this  enunciation  does  not 
betray  to  us  what  matter  actually  is,  because  our  knowledge 
regarding  its  origin  and  basic  structure  is  as  yet  very  incom- 
plete. It  is  permissible,  however,  to  speak  of  certain  definite 
forms  of  matter  as  bodies,  and  of  matter  of  a  rather  conscript 
character  as  substances. 

Science  deals  with  the  structure  and  behavior  of  matter  in 
accordance'  with  well  established  physical  and  chemical 
laws.  This  method  of  investigation  creates  a  sharp  dividing 
line  between  true  scientific  knowledge  and  that  of  a  more  or 
less  speculative  kind.  Accordingly,  the  person  who  pursues 
this  means  of  discovering  and  analyzing  hitherto  unknown 
facts  pertaining  to  matter  may  be  termed  a  scientist.  It  is 
obvious,  however,  that  scientific  knowledge  may  also  be 
2  17 


18  GENERAL    PHYSIOLOGY 

acquired  from  the  writings  of  persons  who  have  been  actively 
engaged  in  this  kind  of  work.  No  difference  can  be  noted 
between  a  student  of  this  character  and  one  seeking  to 
obtain  a  knowledge  of  Latin  or  Greek.  Their  methods  of 
study  are  practically  identical,  although  the  quality  of  their 
knowledge  differs.  Admittedly,  however,  such  a  student 
cannot  justly  be  called  a  scientist  in  spite  of  the  fact  that  the 
acquisition  of  scientific  data  requires  a  practical  trend  of 
mind  whikshf  will  enable), him ;  to  analyze  the  condition  and 
behavior  of  matter 'in  a'ri  introspective  manner.  Like  the 
caipen^£  £F  piiB&bef  ,r  f  he 'toe  'scientist  derives  his  knowledge 
in  a  practical 'way  in'th'e*  Workshop,  and  does  not  rely  solely 
on  books  and  didactic  expositions  for  his  information. 

Although  it  is  commonly  held  that  all  matter  is  dynamic, 
it  may  be  stated  at  this  time  that  it  presents  itself  to  the 
layman  essentially  in  two  forms :  namely,  as  living  or  organic 
and  as  non-living  or  inorganic  matter.  The  inanimate 
material  is  dealt  with  by  the  sciences  of  physics  and  chemis- 
try, and  the  animate,  by  the  science  of  biology.  Accordingly, 
biology  may  be  defined  as  the  science  which  treats  of  matter 
when  it  is  in  the  living  state.  Strictly  speaking,  however, 
matter  does  not  retain  a  perfectly  static  condition  for  any 
length  of  time.  Even  the  stones  may  be  said  to  live,  because 
their  formation  was  attended  with  definite  creative  proc- 
esses, while  their  apparently  static  existence  is  largely  one 
of  constant  deterioration.  In  fact,  some  even  acquire 
material  and  may,  therefore,  be  said  to  be  growing.  It  is 
true,  however,  that  their  creation  and  growth  present 
certain  details  which  are  not  directly  discernible  in  the  life 
of  an  animal.  A  mass  of  gun  powder  may  be  made  to  burn 
and  explode.  Likewise,  it  is  a  biological  truism  that  an 
animal  is  capable  of  reducing  certain  substances  from  which 
it  then  derives  a  sufficient  amount  of  energy  to  manifest 
life.  It  cannot  be  accepted  as  a  fact  that  these  phenomena 
of  the  inorganic  and  organic  worlds  are  wholly  different  in 
their  nature,  because  the  principle  involved  in  these  proc- 
esses is  obviously  the  same,  although  it  presents  a  somewhat 
different  aspect  in  accordance  with  the  kind  of  matter 
undergoing  the  decomposition. 


LIVING    MATTER  19 

While  this  question  may  be  debated  at  some  length,  the 
beginning  student  should  remember  that  the  biological 
sciences  deal  with  the  phenomena  presented  by  living  matter, 
while  the  abiological  sciences  treat  of  those  manifested  by 
non-living  matter.  Accordingly,  biology  may  be  divided 
first  of  all  into  two  sub-sciences :  namely,  zoology  and  botany. 
The  former  concerns  itself  with  the  gross  appearance  of 
animals,  and  the  latter,  with  that  of  plants.  The  processes 
of  life,  however,  are  the  same  in  both  forms,  and  only  when 
studied  in  a  superficial  manner  can  we  recognize  definite 
differences  between  them. 

We  may  approach  the  study  of  living  matter  from  two 
standpoints,  taking  cognizance  either  of  the  structure  of  its 
different  components,  or  of  their  individual  or  joint  action. 
The  maker  of  a  clock  first  of  all  familiarizes  himself  with 
the  character  and  number  of  the  wheels  and  cogs  which  go  to 
form  the  entire  mechanism  before  he  actually  joins  them  in 
an  attempt  to  discover  how  they  fit  into  one  another  and 
move.  Quite  similarly,  the  study  of  the  heart  or  eye  is 
scarcely  feasible  without  first  having  obtained  a  clear  idea 
regarding  the  general  arrangment  and  structural  details  of 
these  organs.  Accordingly,  we  may  represent  the  scope  of 
biology  as  follows: 

Origin,    development,    and    classification:     (embryology, 
zoology,  and  botany) 


Living 
Matter 


Structure 


General  Morphology  j  £™ 


SnPoiMi  I  Gross  (anatomy) 

I  °P«   ll  \  Minute  (histology) 


T-,       ,.       f  General  Physiology 
Function  <  0       •  i  ™.     •  i 

[  Special  Physiology 

Physiology  is  the  study  of  the  dynamics  of  life.  Its  pur- 
pose is  to  analyze  the  processes  occurring  in  living  matter. 
It  is  true,  however,  that  a  study  of  the  phenomena  of  life 
cannot  well  be  attempted  without  a  knowledge  of  the  abiologi- 
cal sciences,  because  in  many  instances  physiology  consists 
merely  of  a  study  of  the  physics  and  chemistry  of  living 


20  GENERAL   PHYSIOLOGY 

material.  It  need  not  surprise  us,  therefore,  to  find  that  the 
progress  of  physiology  is  closely  interlinked  with  the  develop- 
ment of  the  aforesaid  sciences,  because  every  new  physical 
and  chemical  fact  must  greatly  aid  us  in  throwing  additional 
light  upon  the  behavior  of  living  matter.  Since  the  struc- 
tural sciences,  such  as  anatomy  and  histology,  occupy  a 
much  more  independent  position  in  this  regard,  it  has  been 
possible  to  advance  them  at  a  much  faster  rate  than  physi- 
ology. For  this  reason,  anatomy  was  able  at  an  early  date 
to  assume  a  controlling  influence  in  medical  education,  its 
preponderating  position  having  been  seriously  contested  by 
physiology  only  in  more  recent  years. 

Physiology  belongs  essentially  to  the  nineteenth  century 
and  is,  therefore,  a  comparatively  new  science.  In  spite  of 
its  youth,  however, .  it  presents  a  wealth  of  very  valuable 
and  highly  interesting  information  which  finds  its  most 
direct  application  in  medicine,  an  art  purposing  the  relief  of 
suffering  and  prolongation  of  life.  Its  decidedly  practical 
character,  however,  has  also  gained  for  it  an  unfaltering 
place  in  general  education.  Admittedly,  everybody  ought 
to  attempt  to  gain  a  rather  concise  idea  regarding  the  func- 
tions of  the  different  organs  of  his  body,  so  that  he  may  be  in 
a  more  favorable  position  to  take  care  of  what  has  been 
entrusted  to  him  by  Nature.  In  the  form  of  general  knowl- 
edge, physiology  will  aid  us  very  materially  in  mastering  and 
eradicating  those  forces  of  nature  which  tend  to  enfeeble 
us.  Hence,  its  goal  is  the  welfare  of  mankind. 

Protoplasm. — The  term  protoplasm  is  derived  from  the 
Greek  words  "first  form,"  and  is  applied  in  a  general  way  to 
all  types  of  living  matter.  In  the  words  of  Huxley,  it 
constitutes  the  physical  basis  of  life.  This  material  presents 
itself  as  a  rule  as  a  semi-fluid,  viscous  entity,  possessing  a 
reticulated  appearance,  very  similar  to  that  of  a  cluster  of 
soap-bubbles.  Hence,  however  simple  its  organization  may 
be,  it  always  consists  of  a  relatively  resistant  framework  and 
a  homogeneous  watery  ground  substance. 

When  living  matter  is  subjected  to  chemical  analysis, 
its  constitution  immediately  undergoes  certain  very  funda- 
mental changes  which  soon  make  it  impossible  for  it  to 


LIVING   MATTER  21 

manifest  its  ordinary  processes  of  life.  But  in  spite  of  the 
destruction  of  its  function,  such  ,an  analysis  invariably  proves 
that  it  is  not  composed  of  a  single  substance,  but  of  several, 
which  are  combined  in  such  a  way  that  they  may  interact 
with  one  another  producing  a  chemical  basis  for  life.  Living 
matter  is  somewhat  like  a  proteid,  because  it  always  contains 
the  element  nitrogen.  In  addition,  it  embraces  carbon, 
hydrogen,  oxygen,  sulphur,  and  phosphorus.  Chiefly  by 
admixture,  it  may  also  acquire  calcium,  sodium,  potassium, 
silica,  and  other  elements.  Accordingly,  protoplasm  or 
living  matter  is  made  up  of  six  primary  constituents  to  which 
others  may  be  added  until  it  embraces  sixteen  of  the  elements 
now  known  to  chemists.  We  do  not  know  precisely  how  these 
constituents  are  arranged  to  give  rise  to  protoplasm  and 
hence,  it  is  quite  impossible  at  the  present  time  to  form  it 
artificially  in  a  test  tube.  And  even  if  we  should  succeed 
some  future  day  in  producing  it,  we  would  still  be  confronted 
by  the  problem  of  causing  it  to  undergo  those  peculiar 
changes  which  enable  it  to  evolve  its  characteristic  life 
processes. 

The  Origin  and  Evolution  of  Protoplasm. — While  most 
astounding  discoveries  have  been  made  in  recent  years,  it  is 
very  doubtful  whether  the  origin  and  character  of  the  uni- 
verse will  ever  become  fully  known  to  us.  In  the  abiological 
as  well  as  biological  sciences  the  number  of  known  facts  is 
really  insignificant  in  comparison  with  the  totality  of  still 
unknown  facts.  It  is  questionable  whether  this  relationship 
will  ever  be  reversed.  The  scientist  is  an  optimist  in  this 
regard,  as  may  be  gathered  from  his  attempt  to  explain  the 
formation  of  the  now  perfectly  conscript  bodies  of  the  solar 
system  by  the  nebular  hypothesis.  Likewise,  the  biologist 
has  endeavored  to  elucidate  the  origin  of  life  upon  this  planet 
by  stating  that  protoplasm  originated  at  a  time  when  the 
cooling  of  the  earth  favored  the  molecular  union  between 
several  of  its  most  essential  constituents.  One  globule  of 
living  substance  so  formed  then  gave  rise  to  two,  and  these  in 
turn  to  more  complex  entities.  This  direct  descent  of  the 
species  has  been  more  fully  established  by  Darwin,  but  natur- 
ally, the  facts  presented  by  this  scientist  deal  only  with  the 


22  GENERAL   PHYSIOLOGY 

production   of  definite   modifications   and   new   types   and 
cannot  justly  be  applied  to  the  origin  of  protoplasm. 

The  earth  is  inhabited  by  a  most  perplexing  variety  of 
protoplasmic  entities.  Some  of  these  appear  as  extremely 
small  particles  of  living  matter,  pursuing  a  free  and  independ- 
ent existence,  while  others  are  composed  of  several  particles 
bound  together  to  lead  a  communal  life  of  the  simplest  type. 
When  ascending  the  scale  of  the  Animal  Kingdom  from  the 
protozoa  to  the  amphibia,  reptilia,  fish,  birds  and  mammals, 
we  eventually  arrive  at  comparatively  large  masses  of  living, 
matter  which  owe  their  existence  chiefly  to  the  fact  that 
their  protoplasm  is  divided  into  a  multitude  of  small  globules, 
each  of  which  is  rendered  more  compact  and  stable  by  a  firm 
investing  membrane.  In  the  absence  of  this  organization, 
the  larger  aggregates  of  protoplasm  would  soon  split  into 
much  finer  particles;  in  fact,  even  the  smaller  ones  would  not 
hold  together  without  a  certain  compactness  of  their  surface 
layers.  It  is  essential,  therefore,  that  protoplasm  assume  a 
definite  structural  unity,  and  unity  of  structure  invites 
unity  of  purpose  or  function.  A  particle  of  protoplasm  of 
this  kind  constitutes  a  cell.  Accordingly,  it  must  be  evident 
that  the  term  protoplasm  is  a  very  general  one,  being  syno- 
nymous with  living  matter,  whereas  the  term  cell  refers  to 
conscript  entities  of  protoplasm  capable  of  independent 
function. 

In  the  higher  forms,  many  of  these  globules  of  organized 
protoplasm  are  united  with  others  into  larger  masses  or 
tissues.  Hence,  a  tissue  may  be  defined  as  a  collection  of 
cells  possessing  similar  characteristics.  Again,  several 
tissues  may  be  combined  to  form  an  organ.  Accordingly, 
an  organ  may  be  said  to  be  an  aggregate  of  several  tissues 
fulfilling  a  common  purpose,  but  this  definition  should  not 
convey  the  impression  that  the  functions  of  its  components 
are  absolutely  identical,  because  every  organ  consists  first 
of  all  of  a  framework  or  web,  the  purpose  of  which  is  to  give 
lodgment  to  its  real  functional  element.  In  the  case  of  the 
kidney,  for  example,  it  should  be  noted  that  its  most  import- 
ant structural  unit  is  the  urinary  tubule,  while  its  capsule 
and  internal  septa  of  connective  tissue  merely  lend  compact- 


LIVING    MATTER  23 

ness  to  the  entire  structure.  In  addition,  every  organ  con- 
tains a  certain  amount  of  nervous  tissue  which  controls  its 
function,  and  also  numerous  nutritive  supply  channels  in 
the  form  of  bloodvessels  and  lymphatics. 

It  is  usually  stated  that  bone,  muscle,  and  blood  are  tissues, 
although  it  is  apparent  that  a  muscle,  such  as  the  biceps,  is 
really  made  up  of  several  tissues  and  should,  therefore,  be 
designated  as  an  organ.  It  embraces  a  certain  amount  of 
connective  tissue  in  which  a  large  number  of  muscle  cells  are 
embedded,  and  in  addition  also  nervous  tissue,  bloodvessels, 
and  lymphatics.  Quite  similarly,  it  may  not  be  evident 
at  first  sight  that  blood  is  a  tissue,  because  it  is  made 
up  of  many  independent  cellular  elements  which  are 
suspended  in  a  fluid  intercellular  substance,  known  as 
the  plasma.  Blood  is  as  truly  a  tissue  as  bone,  the  only 
difference  discernible  between  them  being  that  the  latter 
remains  stationary,  whereas  the  former  is  moved  from 
place  to  place. 

When  several  organs  are  joined  to  form  a  single  entity,  we 
obtain  what  is  known  as  an  organism.  Consequently,  an 
organism  may  be  defined  as  a  colony  of  organs,  each  of  which 
unfolds  its  own  peculiar  function  in  order  to  maintain  the 
welfare  of  the  whole.  In  the  highest  forms,  these  organs  are 
arranged  in  groups,  giving  rise4 to  systems,  such  as  the  circu- 
latory, respiratory,  nervous,  digestive,  excretory,  and  gen- 
erative ;  but  while  each  system  embraces  a  number  of  different 
organs,  its  functional  product  is  uniform  in  character. 
This  fact  is  well  illustrated  by  the  circulatory  system  which 
embodies  a  pumping  station  or  heart,  and  a  large  number  of 
membranous  channels  by  means  of  which  the  nutritive  fluids 
are  distributed  to  all  parts  of  the  body.  Likewise,  the 
digestive  system  is  composed  of  several  membranous  recept- 
acles for  the  accommodation  of  the  food,  while  a  series  of 
glandular  organs  furnish  powerful  secretions  which  are 
intended  to  simplify  it.  What  is  true  of  animals  is  also  true 
of  plants.  Thus,  the  roots  hold  the  plant  firmly  in  the 
ground  and  are  chiefly  responsible  for  its  nutrition,  while 
the  stem  gives  lodgment  to  the  leaves  in  which  certain  chemi- 
cal processes  are  effected  under  the  influence  of  sunlight.  The 


24 


GENERAL    PHYSIOLOGY 


flowers  give  rise  to  the  fruits  which  embrace  the  seeds  or 
reproductive  elements. 

This  division  of  labor  is  evident  even  in  the  simplest 
organisms,  such  as  the  paramoecium.  Its  movements  are 
evoked  by  the.  contraction  of  specialized  filaments  of  proto- 
plasm, the  cilia,  while  its  digestive  function  is  instigated  in 
an  indentation  of  the  integument,  the  gullet.  Another 
specialized  mechanism  is  the  contractile  vacuole  which 
appears  to  subserve  the  movement  of  the  intracellular 
material  and  excretion.  Hence,  even  such  simple  protoplas- 
mic entities  as  the  protozoa  are  true  organisms. 

The  Cell. — Whether  protoplasm  is  organized  to  form  a 
single  free-living  entity  or  a  simple  constituent  of  the  tissues 


FIG.  1. — The  structure  of  protoplasm.     An  epidermal  cell  of  the  earth- 
worm.    (After  Butschli.) 

of  the  most  complex  animal  or  plant,  it  always  presents 
certain  structural  and  functional  characteristics.  A  small 
particle  of  living  matter  possessing  sufficient  organization 
to  be  independently  active,  is  known  as  a  cell.  Accordingly, 
it  may  be  said  that  a  cell  represents  the  simplest  type  of 
individuality  of  living  substance.  It  constitutes  a  unit  in 
structure  and  function.  As  originally  applied,  the  term 
protoplasm  referred  merely  to  the  viscous  ground-substance 


LIVING   MATTER 


25 


of  cells.  We  now  know,  however,  that  this  constituent  is 
quite  unable  to  unfold  its  life  processes  unless  mixed  with 
certain  other  elements  which  are  briefly  designated  as  nuclear 
material.  In  analogy  with  the  general  conception  that  a 
cell  is  a  walled  space,  similar  in  its  outline  to  those  forming 
the  honeycomb  or  nests  of  certain  insects,  it  has  also  been 
held  that  these  fine  globules  of  protoplasm  are  invariably 
surrounded  by  a  delicate  membranous  capsule.  This  is  not 
always  the  case,  because  such  organisms  as  the  amoeba  do 


Attraction-sphere  enclosing  two  centrosomes 


Plastids  lying  in  the 
cytoplasm 


Nucleus 


Karyosome, 
net-knot,  or 
chromatin- 

nucleolus 


Vacuole 


Passive  bodies  (meta- 
plasm  or  para  plasm) 
suspended  in  the  cy- 

toplasmic  raeshwork 


FIG.  2. — Diagram    of    a    cell.     (Wilson.) 

not  possess  a  distinct  enveloping  membrane.  In  the  face 
of  these  still  debated  questions,  it  seems  advisable  to  obtain 
first  of  all  a  clear  conception  of  the  structure  of  one  of  the 
more  familiar  types  of  cells,  for  example,  of  those  composing 
the  acini  of  the  liver,  pancreas,  or  salivary  glands. 

A  cell  of  this  kind  is  invested  by  a  clearly  differentiated 
wall,  while  its  interior  is  occupied  by  a  clear,  homogeneous, 
viscous  substance  which  is  known  as  cytoplasm.  Somewhere 
in  this  ground-material  is  embedded  a  dense,  dark  object, 


26 


GENERAL   PHYSIOLOGY 


generally  rounded  in  outline,  which  is  known  as  a  nucleus. 
Within  the  latter  is  often  found  a  still  smaller  globule  of 
dark-staining  material  which  is  designated  as  a  nucleolus. 
Under  the  low  power  of  the  microscope  the  ground  substance 
exhibits  a  homogeneous  watery  consistency,  which,  however, 
becomes  froth-like  when  subjected  to  high  magnifications. 
Such  a  formation  may  be  reproduced  artificially  by  mixing 


FIG.  3. — The  functional  relation  of  the  cytoplasm  and  nucleus.  A. 
An  amoeba  divided  into  a  nucleated  and  non-nucleated  portion.  B.  The 
same  portion  after  an  interval  of  eight  days.  (After  Hofer.) 

a  drop  of  oil  with  a  small  amount  of  cane  sugar  or  potassium. 
All  cells  are  said  to  contain  nuclear  material,  although  it 
may  not  always  appear  in  the  form  of  a  sharply  differentiated 
body,  but  as  minute  particles  scattered  through  the  cyto- 
plasm. At  all  events,  the  nucleus  is  generally  considered  as 
being  absolutely  essential  to  the  life  of  the  cell,  because  any 
mass  of  cytoplasm  which  has  been  deprived  of  this  constitu- 
ent, soon  loses  its  function  and  undergoes  degenerative 
changes.  This  fact  may  be  proved  very  easily  by  dividing 
an  amceba  into  two  parts  in  such  a  way  that  its  nucleus  is 
left  entirely  in  one  of  these  segments.  The  nucleated  portion 


LIVING    MATTER  27 

regenerates  very  promptly,  whereas  the  denucleated  part 
soon  ceases  to  move  and  to  ingest  food. 

The  cytoplasm  of  a  cell  also  contains  certain  formed  ele- 
ments, representing  material  ready  for  assimilation  and  excre- 
tion. In  addition,  it  may  embrace  certain  accidental 
admixtures,  such  as  pieces  of  shells  and  granules  of  sand. 
These  admixtures  are  most  easily  recognized  in  the  unicellular 
organisms  which  catch  the  particles  of  food  by  engulfing 
them. 

The  Chemical  Composition  of  the  Cell. — The  chemical 
constitution  of  a  cell  cannot  be  ascertained  during  its  life, 
because  the  analytical  procedures  employed  at  the  present 
time  bring  its  functional  changes  to  a  standstill.  It  should 
also  be  remembered  that  certain  varieties  of  cells  may  contain 
a  type  of  material  which  is  not  present  in  others.  Such  an 
inconstant  constituent  is  the  glycogen  of  the  liver  cell. 
But,  disregarding  these  minor  differences  for  the  moment,  it 
may  be  said  that  protoplasm  contains  about  75  per  cent,  of 
water  and  25  per  cent,  of  solids.  Among  the  latter  should 
be  mentioned : 

A.  Organic  Material,     (a)  Proteins. — These  are  the  most 
constant  and  important  constituents,  and  are  present  in  the 
cytoplasm  as  well  as  in  the  nucleus. 

(6)  Fats  and  Lipoids. — Besides  ordinary  fats,  protoplasm 
also  contains  certain  substances  which  are  soluble  in  alcohol 
and  ether.  They  are  known  as  lipoids.  One  of  the  most 
important  bodies  of  this  kind  is  cholesterol.  When  they 
embrace  phosphorus,  they  are  designated  as  phosphatides. 
Lecithin  is  the  most  important  member  of  this  group. 

(c)  Carbohydrates. — These  organic  substances  are  not 
found  as  original  and  free  constituents  of  the  cells.  They 
may  be  present  in  glycoproteids  and  cerebrosides. 

B.  Inorganic    Material,     (a)  Water    and    Salts. — As    has 
just  been  stated,  water  constitutes  about  three-fourths  of 
the  entire  bulk  of  protoplasm.     In  addition  to  the  six  princi- 
pal elements  mentioned  above,  it  may  also  contain  sodium, 
potassium,  magnesium,   calcium,  iron,  and  at  times  even 
iodin,   manganese,   copper,   zinc,  barium,   and  silica.     The 
proportion  of  these  elements  differs  greatly  in  different  cells. 


28 


GENERAL   PHYSIOLOGY 


The  Size  and  Shape  of  the  Cell. — Inasmuch  as  living  mat- 
ter appears  either  in  the  form  of  individual  entities  or  as 
units  of  tissues,  it  may  rightly  be  assumed  that  the  size  and 
shape  of  the  different  cells  vary  considerably.  It  may  also 
be  taken  for  granted  that  their  fundamental  shape  is  round, 
or  nearly  so,  and  that  they  become  polyhedral  only  when 
combined  with  others  to  form  larger  masses.  Likewise,  it 
may  be  concluded  that  they  cannot  attain  a  very  large  size, 
because  the  ordinary  physical  forces  do  not  suffice  to  hold 


FIG.  4. — Epithelial  cells  in  the  web  of  the  frog. 

their  substance  together.  By  far  their  largest  number 
remains  below  the  range  of  ordinary  vision,  and  very  few 
attain  dimensions  that  may  be  expressed  in  terms  of  milli- 
meters (1) .  Furthermore,  while  no  cell  is  absolutely  dormant 
when  living,  they  exhibit  different  degrees  of  motile  power. 
Thus,  it  is  evident  that  the  cells  forming  the  ordinary  tissues 
and  organs  of  our  body  are  fixed,  •  whereas  the  free-living 
cells,  such  as  constitute  the  unicellular  organisms,  are 
capable  of  shifting  their  protoplasm  and  of  moving  from 
place  to  place.  This  form  of  movement  which  is  character- 
ized as  amoeboid,  is  most  clearly  betrayed  by  such  organisms 
as  the  amoeba  and  leukocyte.  Again,  jnany  organisms 
consisting  of  only  one  cell,  may  move  by  means  of  accessory 
structures,  such  as  flagellse  and  cilia.  These  filamentous 


LIVING    MATTER 


29 


processes  are  motile,  while  the  principal  mass  of  the  cell 
remains  inactive. 

1.  The  unit  of  histological  measurement  is  the  micro-millimeter. 
It  is  indicated  by  the  symbol  /*.  It  will  be  remembered  that  a  meter 
(39.3  inches)  consists  of  100  centimeters;  and  each  centimeter  (cm.) 
10  millimeters  (mm.)  A  micro-millimeter  is  the  Kooo  part  of  a 


FIG.  5. — Amoeba. 


(From    Calkins'    "Biology"    Courtesy   of  Henry  Holt 
&  Co.,  Publishers.) 


millimeter.  Structures  of  so  small  a  size  must  be  subjected  to 
microscopic  vision  in  order  to  render  them  perceptible  to  the 
eye,  a  magnification  of  from  300  to  400  diameters  being  required  to 
accomplish  this  end.  But  we  also  recognize  in  an  indirect  way  certain 
active  particles  which  cannot  be  seen  even  with  the  aid  of  a  microscope. 
They  are  known  as  ultra-microscopic  entities. 

In  order  that  the  student  may  from  a  clear  conception  of  these 
measurements,  he  should  be  in  possession  of  a  familiar  object  for 
comparison.  For  example,  the  shaft  of  a  human  hair  possesses  a 

diameter  of  0.08  mm.  or  about  5^  of  an  inch.     Likewise,  the  shaft 

oOO 

of  the  hair  from  the  fur  of  a  rabbit  has  a  diameter  of  0.025  mm.  which 
value  equals  about  JQ^Q  of  an  inch.  Its  tip,  on  the  other  hand,  measures 

only  about  0.001  mm.  or      QQ    of  an  inch.     Accordingly,  since  the 

red  corpuscle  of  the  human  blood  is  7fj.  or  0.007  mm.  in  diameter, 
its  cross-section  would  be  about  7  times  as  large  as  that  of  the  tip  of 
a  rabbit's  hair. 


CHAPTER  II 
GENERAL    PHENOMENA    OF    LIFE 

The  Relationship  Between  Animals  and  Plants. — The 
processes  of  life  may  be  investigated  in  two  ways :  namely,  by 
chemical  means  and  physical  means.  Thus,  we  may  en- 
deavor to  discover  the  composition  of  living  matter  and 
attempt  to  detect  the  changes  which  arise  therein  in  the 
course  of  its  activities.  Living  matter  is  never  at  a  standstill, 
but  assimilates  and  dissimilates  substances  constantly. 
The  tracing  of  these  through  the  organisms  is  one  of  the 
chemical  problems  pertaining  to  life.  Again,  we  may  study 
living  matter  from  the  physical  standpoint,  and  note  the 
mechanics  of  its  behavior.  Living  substance  liberates 
different  energies:  namely,  mechanical  energy,  heat,  light, 
and  electricity.  To  portray  these  in  a  realistic  manner  would 
be  another  way  of  ascertaining  the  cause  and  characteristics 
of  life.  But  whichever  method  may  be  followed,  it  cannot 
yield  favorable  results  unless  amplified  by  the  other. 

We  have  been  accustomed  in  recent  years  to  regard  organ- 
isms as  mechanisms  which  act  in  accordance  with  definite 
chemical  and  physical  laws.  In  other  words,  our  study  of  the 
processes  of  life  has  been  directed  along  mechanistic 
channels,  whereas  living  entities  were  formerly  regarded  as 
unique  in  character,  because  they  presented  many  as  yet 
quite  incomprehensible  manifestations.  This  vitalistic  view 
has  not  aided  scientific  progress  very  materially,  because  a 
conception  which  admits  right  at  the  start  our  impotence 
to  cope  with  a  certain  problem,  is  by  no  means  the  most 
advantageous  to  select  as  a  guiding  principle. 

It  must  be  clearly  recognized  that  matter  is  indestruc- 
tible, and  that  all  energy  is  conserved.  Supposing  that  we  are 
in  possession  of  a  certain  inorganic  substance  which  may  be 
split  chemically  into  its  several  components,  we  should  not 

30 


GENERAL    PHENOMENA    OF   LIFE  31 

assume  that  this  substance  is  then  absolutely  lost.  It 
merely  assumes  a  different  aspect  for  a  certain  period  of 
time,  and  may  again  be  built  up  into  its  original  form  later  on. 
Furthermore,  its  components  may  severally  enter  new 
combinations.  This  law  of  the  conservation  of  matter  and 
energy  is  also  applicable  to  organic  material.  Thus  we  find 
that  the  animals  consume  different  complex  substances 
which  are  reduced  into  their  simple  components  and  assimi- 
lated with  liberation  of  definite  forms  of  energy.  They 
finally  leave  the  body  as  waste  products.  Clearly,  therefore, 
these  substances  are  not  lost  but  are  merely  transformed  into 
simpler  ones. 

It  is  also  apparent  that  a  certain  functional  reciprocity 
exists  between  animals  and  plants,  because  the  life  of  one  sus- 
tains that  of  the  other.  This  statement,  however,  holds 
true  only  for  those  species  which  one  would  naturally  select 
for  the  purpose  of  illustrating  this  particular  point,  because 
certain  varieties  of  plants,  such  as  the  fungi,  are  not  in  pos- 
session of  coloring  material,  and  are,  therefore,  quite  unable 
to  transpose  material  for  fuel.  In  fact,  the  lowest  living 
entities  frequently  present  so  similar  a  structure  and  func- 
tion that  one  might  be  justified  in  classifying  them  either 
as  animals  or  plants.  A  green  plant  is  able  to  construct  its 
complex  material  from  the  simplest  substances,  such  as  water, 
carbon"  dioxid,  and  different  inorganic  salts.  An  animal  also 
requires  certain  organic  material,  although  it  is  quite  unable 
to  form  these  from  elementary  inorganic  substances.  Con- 
sequently, the  animal  must  acquire  its  fuel  in  a  preformed 
state,  and  since  the  plants  are  capable  of  producing  car- 
bohydrates, fats  and  proteids  from  elementary  substances, 
they  become  the  most  important  source  of  nutritive  supply  to 
the  animals.  Thus,  it  may  be  said  that  the  animals  are  the 
parasites  of  the  plants,  because  their  existence  depends  in  a 
large  measure  upon  the  food  prepared  for  them  by  the  latter. 
As  has  been  stated  above,  the  fungi  which  contain  no  chloro- 
phyl,  are  in  a  similar  position,  because  they  must  derive 
their  requirement  of  carbon  from  existing  organic  material. 

It  is  evident  that  the  plants  are  an  essential  factor  in  the 
life  of  animals.  The  reverse,  however,  is  not  true,  because 


32  GENERAL   PHYSIOLOGY 

water  and  inorganic  salts  are  abundant  in  nature  and  carbon 
dioxid  may  be  obtained  from  other  sources  than  the  expired 
air  of  animals.  The  excretory  products  of  the  animals, 
however,  aid  the  plants  at  times  in  building  up  their  sub- 
stances. For  example,  it  is  a  well  known  fact  that  the  car- 
bon dioxid  emitted  in  the  expiratory  air  of  animals  or  as 
products  of  general  combustions,  is  immediately  made  use 
of  by  the  plants  in  their  constructive  processes.  This 
synthetic  power  of  the  plants  is  of  greatest  importance, 
because  it  relieves  the  atmosphere  of  this  admixture  which 
if  allowed  to  accumulate  in  considerable  amounts,  would 
eventually  endanger  the  life  of  all  animals.  Again,  the 
plants  liberate  oxygen  which  is  inspired  later  on  by  animals, 
but  this  gas  does  not  arise  in  the  course  of  their  respiratory 
processes,  because  plants  inhale  oxygen  and  exhale  carbon 
dioxid.  It  is  metabolic  oxygen  generated  as  a  by-product 
during  the  formation  of  the  complex  constituents  of  the 
plants  from  simple  substances.  Sunlight  and  chlorophyl  are 
the  factors  absolutely  essential  for  this  constructive  process. 
When  regarded  in  a  general  way,  it  may  be  said  that  the 
plants  form  potential  material  for  the  animals,  because  they 
abstract  simple  substances  from  the  earth  and  unite  them  into 
complex  foodstuffs,  such  as  the  starches  and  the  proteins. 
Plants  are  devoured  by  animals,  and  some  animals  in  turn 
are  devoured  by  others.  These  complex  substances  are 
then  broken  down  by  the  latter  under  liberation  of  energy 
and  are  excreted;  in  fact,  the  animal  as  a  whole  eventually 
splits  up  into  its  simple  components,  permitting  them  to  be 
again  incorporated  in  the  earth.  Strangely  one  is  here 
reminded  of  the  words  of  Hamlet  which  read : 

"Imperial  Caesar,  dead  and  turned  to  clay 
Might  stop  a  hole  to  keep  the  wind  away, 
Oh  that  that  earth  which  kept  the  world  in  awe 
Should  patch  a  wall,  to  expel  the  winter's  flaw." 

This  transmutation  of  matter,  however,  cannot  be  con- 
sidered as  being  wholly  constructive  in  the  plants  and  destruc- 
tive in  the  animals,  because  many  animal  tissues  are  capable 
of  synthetizing  simple  substances  into  very  complex  bodies. 
For  example,  it  is  a  well  known  fact  that  the  proteins  of 


GENERAL   PHENOMENA    OF   LIFE  33 

plants  are  broken  down  in  the  animal  into  their  amino-acid 
constituents,  and  that  these  building  stones  are  again  united 
later  on  into  the  protein  substances  of  the  body.  In  many 
instances,  these  body-proteins  are  quite  'different  from 
those  ingested  as  food,  but  these  syntheses  are  after  all 
merely  minor  variations  in  the  general  process  of  the  trans- 
formation of  matter. 

Oxidation. — Living  substance  is  never  in  a  state  of  complete 
inactivity.  It  grows;  it  moves;  it  secretes;  and  gives  rise 
to  various  other  changes  which  necessitate  the  production 
of  energy.  Where  is  this  energy  derived  from?  Evidently 
from  a  definite  source  within  the  protoplasm,  because  the 
substances  entering  into  its  composition  undergo  certain 
changes  in  the  presence  of  oxygen.  During  these  processes 
a  certain  form  and  amount  of  energy  is  liberated  which 
renders  the  protoplasm  dynamic.  Contrariwise,  any  mass 
of  protoplasm  which  is  unable  to  instigate  these  changes, 
must  cease  being  a  living  entity. 

Oxidation,  therefore,  may  be  defined  as  a  chemical  union 
of  oxygen  with  any  other  substance.  Let  me  illustrate  this 
statement  by  an  example.  A  match  is  ignited  by  striking 
its  head  against  a  rough  surface.  The  friction  produced 
thereby  causes  the  phosphorus,  an  element  possessing  a 
great  affinity  for  oxygen,  to  unite  with  this  particular  con- 
stituent of  the  atmospheric  air.  In  addition,  the  head  of  a 
match  is  usually  covered  with  a  layer  of  sulphur  which 
combines  with  oxygen  at  a  much  higher  temperature  than 
phosphorus.  Red  lead,  niter,  and  gum  or  glue  are  added  to 
the  impregnation  fluid  for  the  purpose  of  liberating  oxygen  at 
the  beginning  of  this  process  and  to  bind  all  these  substances 
together.  Eventually  the  wood  is  ignited.  The  latter  is 
composed  in  largest  part  of  the  element  carbon,  and  under- 
goes a  much  slower  form  of  oxidation  than  the  other  constit- 
uents of  the  match.  During  this  process,  a  considerable 
amount  of  heat  is  evolved  which  may  be  made  use  of  to 
produce  work.  Quite  similarly,  the  oxidation  of  coal  liber- 
ates heat  which  may  be  employed  to  boil  water  and  to  gener- 
ate steam  for  mechanical  purposes.  This  form  of  energy 
is  the  direct  basis  of  work.  Reference  should  also  be  made 


34  GENERAL    PHYSIOLOGY 

at  this  time  to  certain  oxidations  which  do  not  evolve  an 
appreciable  amount  of  heat.  For  example,  if  an  iron 
nail  is  placed  in  a  dish  containing  a  small  quantity  of  water, 
it  will  presently  become  covered  with  rust.  This  deposit 
is  composed  of  iron  oxide,  a  product  of  the  union  of  iron 
with  oxygen. 

Inasmuch  as  carbon  is  the  most  important  constituent  of 
things  that  possess  life,  or  organic  material,  the  oxidations 
in  cells  must  naturally  strive  to  reduce  the  carbon  combina- 
tions into  simpler  bodies.  It  is  easily  observed  that  the 
union  of  oxygen  and  carbon  gives  rise  to  a  gaseous  product 
which  is  known  as  carbon  dioxid  or  carbonic  acid  gas. 
Another  important  component  of  organic  matter  is  nitrogen, 
but  this  element  is  an  inert  gas  and  does  not  support  com- 
bustions. The  only  other  important  constituent  of  such 
matter  is  hydrogen.  When  this  gas  combines  with  oxygen, 
water  is  formed.  Thus,  the  burning  candle  liberates  carbon 
dioxid  from  its  carbon,  and  water  vapor  from  the  hydrogen  of 
the  wax.  During  these  processes  a  certain  amount  of  heat 
is  evolved,  its  rate  of  discharge  varying  greatly  with  the 
character  of  the  material. 

Oxidation  in  Cells. — The  destiny  of  living  matter  is  to 
liberate  energy  and  to  produce  work.  In  this  regard  it 
does  not  differ  from  any  other  material.  The  manner  in 
which  this  end  is  accomplished  is  based  upon  a  common  prin- 
ciple, namely,  that  of  oxidation.  As  in  the  cases  of  the 
match,  coal  and  candle,  the  plants  and  animals  cleave  the 
complex  substances  with  the  liberation  of  carbon  dioxid  and 
water.  This  decomposition  is  as  important  to  one  as  it  is 
to  the  other,  because  it  gives  rise  to  an  evolution  of  heat,  and 
heat  is  the  source  of  all  forms  of  work. 

What  is  true  of  protoplasm  as  a  whole  is  true  of  the  cells, 
whether  free-living  or  combined  with  others  into  tissues  and 
organs.  Each  is  in  possession  of  a  certain  store  of  material 
which  it  must  burn  up  in  order  to  fulfill  its  purpose  in  life. 
To  be  sure,  the  protoplasm  of  the  different  cells  presents  cer- 
tain variations  in  its  chemico-physical  constitution.  These 
differences  between  the  cells  lead  to  differences  in  their 
functional  products.  Thus,  while  one  cell  may  furnish 


GENERAL   PHENOMENA    OF   LIFE  35 

mechanical  energy  in  the  form  of  motion,  another  may  yield 
a  secretion,  but  quite  irrespective  of  their  final  products,  the 
principle  involved  in  these  activities  remains  the  same  in  all 
cases.  Every  cell  must  sacrifice  its  stored  energy  in  order 
to  live,  although,  correctly  speaking,  this  decomposition 
cannot  be  called  a  sacrifice,  because  by  following  this  path 
the  cell  merely  fulfills  the  laws  of  the  forces  which  created  it. 

Metabolism. — The  constant  liberation  of  energy  by  the 
cell  must  be  compensated  for.  As  living  matter  is  unable  to 
create  its  substances,  it  must  get  them  from  some  external 
source,  i.e.,  from  the  medium  in  which  it  lives.  Here  various 
substances  are  held  in  readiness  for  it  which  it  may  acquire 
and  utilize.  All  these  substances  contain  energy,  but  energy 
in  a  resting  or  potential  state.  It  becomes  the  duty  of  the 
cell  to  convert  this  potential  energy  into  its  dynamic  or 
kinetic  form. 

This  statement  may  lead  us  to  infer  that  the  cell  cannot  go 
on  decomposing  materials  indefinitely.  It  must  also  acquire 
new  substances  to  take  the  places  of  those  destroyed.  The 
former  process  is  designated  as  dissimilation  or  catabolism 
and  the  latter,  as  assimilation  or  anabolism.  Both  together 
constitute  the  process  of  metabolism.  Accordingly,  it  may 
be  said  that  a  cell  is  able  to  retain  its  position  as  the  structural 
and  functional  unit  of  living  matter  only  as  long  as  its  meta- 
bolic processes  are  rigidly  controlled  by  it.  A  cell  which  in 
consequence  of  some  inherent  cause  fails  to  metabolize, 
must  cease  its  function  and  lose  its  value  as  a  structural 
unit.  But,  naturally,  this  result  may  also  follow  an  inability 
on  the  part  of  the  cell  to  obtain  a  sufficient  and  proper  store 
of  materials  from  a  medium  which  is  in  an  impoverished 
condition.  Although  cells  are  capable  of  adapting  them- 
selves to  environmental  changes,  their  power  of  adjustment  is 
limited  and  death  must  follow  all  variations  of  extraordinary 
character. 

The  manner  in  which  cells  acquire  their  nutritive  material 
and  discharge  their  waste  products,  differs  in  accordance 
with  their  general  structure.  Single  organisms  usually 
take  them  directly  into  their  interior.  Digestion  and  assimi- 
lation then  follow.  The  multicellular  organisms,  on  the 


36  GENERAL   PHYSIOLOGY 

other  hand,  are  in  possession  of  special  colonies  of  cells  in  the 
form  of  organs,  to  which  the  functions  of  ingestion  and  diges- 
tion are  severally  assigned.  From  the  standpoint  of  the 
general  tissue  cells,  digestion  may  be  said  to  be  extra-cellular, 
because  the  nutritive  substances  are  brought  to  them  com- 
pletely predigested.  The  cells,  however,  are  able  to  assimi- 
late these  simplified  substances  and  to  utilize  them  by  virtue 
of  a  certain  inherent  chemical  power.  The  same  statement 
may  be  made  regarding  the  excretory  substances.  The 
single  free-living  cells  discharge  their  waste  directly  into 
the  medium,  whereas  the  more  complex  organisms  are  in 
possession  of  special  organs  which  are  set  aside  for  the  purpose 
of  eliminating  the  useless  material. 

Food. — The  conversion  of  potential  into  kinetic  energy 
continues  only  as  long  as  nutritive  substances  are  available 
for  purposes  of  assimilation  and  dissimilation.  A  substance 
which  is  instrumental  in  furthering  the  growth  of  the  cell 
and  serves  as  an  aid  in  its  oxidations  to  furnish  energy,  is 
designated  as  a  food.  This  definition  is  sufficiently  embrac- 
ing to  include  water  which  to  all  intents  and  purposes  is  not 
a  fuel,  although  it  possesses  a  distinct  value  as  a  food.  A  simi- 
lar difficulty  arises  in  the  case  of  the  mineral  salts  which  are 
necessary  constituents  of  cells,  but  cannot  singly  be  burned  up 
to  liberate  energy.  The  proteins  are  not  only  essential  consti- 
tuents of  the  cells,  but  also  give  rise  to  energy  on  oxidation. 
Oxygen  occupies  a  unique  position,  because  it  is  not  a  fuel, 
but  merely  aids  in  converting  certain  substances  into  different 
forms  so  that  it  is  possible  to  derive  energy  from  them. 

Not  in  complete  analogy  with  the  plants,  the  animals 
usually  ingest  mixtures  of  nutrient  substances,  such  as  are 
represented  by  potatoes,  beans,  peas,  bread,  butter,  meat, 
etc.  The  substances  which  singly  enter  into  the  formation 
of  a  nutrient  material,  are  known  as  food-stuffs.  Accordingly, 
a  food-stuff  may  be  defined  as  a  nutrient  substance  of  definite 
chemical  composition.  The  five  food-stuffs  which  may  enter 
into  the  formation  of  a  food,  are  water,  salts,  carbohydrates, 
fats,  and  proteins.  Thus,  a  potato  may  be  said  to  contain 
several  food-stuffs,  for  example,  water,  salts,  carbohydrates 
in  the  form  of  starch,  and  a  small  amount  of  protein. 


CHAPTER  III 
GENERAL  CONDITIONS  OF  LIFE 

Animate  and  Inanimate  Material. — Inasmuch  as  physi- 
ology is  the  study  of  the  dynamics  of  life,  it  seems  advisable 
at  this  time  to  make  brief  inquiry  into  the  characteristics 
presented  by  living  matter.  If  the  layman  is  asked  to  tell 
whether  or  not  a  thing  is  living,  he  first  seeks  to  discover  some 
evidence  of  motion,  and  if  the  latter  is  not  forthcoming 
spontaneously,  he  touches  the  object  in  an  attempt  to  call 
forth  a  reaction  by  stimulation.  A  scientific  differentiation 
between  inorganic  and  living  organic  bodies  is  usually  made 
upon  morphological,  genetic,  physical,  and  chemical  grounds. 
It  is  usually  stated  that  inorganic  objects  possess  definite 
geometric  proportions,  while  organic  ones  do  not.  This 
point,  however,  is  not  well  taken,  because  organisms  with 
geometric  contours  are  not  uncommon.  Reference  might  be 
made  at  this  time  to  the  radiolaria,  the  calcarious  envelopes 
of  which  are  laid  down  along  definite  straight  and  curved 
lines.  Upon  genetic  grounds  it  may  be  held  that  organisms 
always  originate  from  organisms,  but  this  difference  remains 
real  only  if  we  adhere  to  the  metaphysical  conception  of  the 
origin  of  life  and  absolutely  deny  that  it  will  ever  be  possible 
to  produce  living  matter  artificially. 

It  is  also  stated  that  living  matter  possesses  the  properties 
of  irritability  and  contractility,  while  inorganic  material 
does  not.  Let  us  take  a  small  mass  of  gunpowder  and  spread 
it  out  in  the  form  of  a  long  and  narrow  band.  On  igniting  it, 
an  explosive  reaction  follows,  during  which  the  powder  is 
progressively  consumed.  The  amount  of  energy  evolved 
during  this  reaction  is  infinitely  greater  than  that  liberated 
by  any  living  organic  material  of  equal  size.  Since  our 
conception  of  irritability  and  conductivity  is  derived  from 
the  study  of  animal  tissues,  it  may  be  somewhat  difficult  to 
recognize  a  distinct  similarity  between  this  process  and  those 

37 


38  GENERAL    PHYSIOLOGY 

taking  place  in  the  inorganic  world.  In  reality,  however, 
there  is  no  difference  unless  it  is  one  of  generalities. 

Chemistry  tells  us  that  living  organic  matter  embraces 
certain  bodies,  the  complexity  of  which  greatly  exceeds  that 
of  any  inorganic  material.  Reference  is  had  here  more 
particularly  to  the  proteins  which  only  living  matter  is  able 
to  cleave  and  to  assimilate.  Even  this  difference  will  no 
longer  be  available  after  a  way  has  been  found  to  produce 
protein  substances  artificially.  One  difference,  however, 
will  always  remain  clearly  recognizable  and  that  is  the 
specific  metabolic  function  of  living  matter.  Life  requires  a 
constant  succession  of  metabolic  changes  involving  complex 
organic  materials.  It  is  not  possible  to  reproduce  these 
with  inorganic  nor  with  dead  organic  material. 

Peculiarities  of  Living  Matter. — In  a  general  way,  it  may 
suffice  to  state  that  living  matter  presents  certain  fundamen- 
tal characteristics  which  may  be  classified  under  the  following 
headings : 

(a)  Irritability. — The   physical   and  chemical  constitution 
of  protoplasm  is  such  that  it  permits  outside  influences  to  be 
received  by  it.     These  influences  are  absolutely  essential  to 
it,  because  they  form  the  stimulus  which  keeps  the  intracellu- 
lar  machinery  in  motion.     While  this  is  a  perfectly  definite 
phenomenon,  all  inquiries  regarding  the  nature  or  .causes 
underlying  this  property  of  protoplasm  must  remain  without 
answer,  because  the  minute  processes  going  on  in  cells  are 
as  yet  not  fully  understood.     From  this  brief  statement  it 
may  be  gathered  that  living  matter  must  fail  to  function 
properly  when  these  stimuli  cease  or  when,  in  consequence  of 
some  inherent  difficulty,  it  becomes  non-receptive  to  them. 

(b)  Conductivity. — Whenever  protoplasm  is  subjected  to  a 
stimulus,  a  local  reaction  follows  which,  however,  does  not 
remain  confined  to  the  locality  first  affected,  but  gradually 
spreads  from  here  to  its  more  distant  regions.     In  this  way,  a 
wave  of  irritability  or  excitability  is  produced  which  extends 
in  all  directions  through  its  mass.     Some  forms  of  protoplasm 
are  more  irritable  and  conductile  than  others.     The  tissue 
which  shows  these  peculiarities  in  the  most  unmistakable  man- 
ner, forms  the  nerves  and  their  distal  and  central  end-stations. 


GENERAL    CONDITIONS    OF    LIFE 


39 


(c)  Contractility. — The  wave  of  excitation  traversing  pro- 
toplasm invariably  gives  rise  to  some  sort  of  a  reaction. 
This  reaction  is  most  plastically  portrayed  by  motion. 
Inasmuch  as  protoplasm  contains  contractile  material,  a 
stimulus  applied  to  it  must  cause  it  to  shift  its  molecular 
constituents  in  such  a  way  that  the  entire  mass  assumes  a 
.different  shape  and  even  moves  from  place  to  place.  Clearly, 
a  sharp  distinction  should  here  be  drawn  between  the  phe- 
nomenon of  contraction  as  observed  by  physiologists  and 


FIG.  6. — Actinosphserium.     A,  position  of  rest;  B,  position  of  stimulation. 

(Verworn.) 

that  noted  by  physicists.  When  an  iron  bar  contracts  in 
cooling,  its  volume  is  reduced  in  all  directions.  An  organism 
contracting,  however,  merely  changes  its  form  and  position, 
but  cannot  gain  or  lose  sufficient  material  to  change  its 
volume. 

(d)  Metabolism. — It  has  been  stated  above  that  protoplasm 
cannot  retain  its  fundamental  characteristics  unless  conr 
stantly  supplied  with  material  from  which  it  may  derive  a 
certain  amount  of  energy.     The  liberation  of  this  energy  is 
accomplished  by  means  of  an  elaborate  series  of  mechanical 
and  chemical  processes. 

(e)  Reproduction. — Living  matter  is  unstable.     It  decom- 
poses but  to  recompose,  and  recomposes  but  to  decompose. 
Hardly  for  a  moment  does  it  remain  precisely  in  the  same 


40  GENERAL   PHYSIOLOGY 

condition.  Besides,  it  undergoes  gradual  senile  changes 
which  eventually  destroy  it  altogether.  Even  barring 
accidents,  its  life  cycle  is  very  brief.  Scarcely  has  it  fully 
unfolded  its  processes  when  it  begins  to  retrogress  and  to  show 
signs  of  impaired  function.  It  follows  that  some  provision 
must  be  made  for  the  replacement  of  these  senile  living 
entities  by  new  ones,  otherwise  life  upon  the  earth  would 
soon  become  extinct.  Nature  has  taken  all  possible  precau- 
tions to  protect  living  matter  against  such  an  eventuality. 
The  perpetuation  of  the  species  is  its  highest  law  which  it 
defends  with  a  perfectly  amazing  prodigality. 

The  function  of  reproduction  should  be  clearly  distin- 
guished from  growth  and  regeneration.  Whenever  the  ingo 
exceeds  the  outgo,  the  organism  accumulates  material.  It 
grows  and  its  weight  increases.  This  formation  of  storative 
protoplasm  out  of  the  food  is  most  intense  during  the  early 
periods  of  the  existence  of  the  organism.  Somewhat  later 
an  accurate  balance  is  established,  which  finally  gives  way 
to  an  increased  expenditure  and  corresponding  shrinkage  of 
the  protoplasm.  It  may  also  happen  that  an  organism 
loses  one  of  its  parts  entirely.  In  many  instances  this  part 
may  again  be  reformed.  The  processes  of  growth  and  re- 
generation, however,  invariably  concern  one  and  the  same 
organism  and  possess  no  direct  bearing  upon  the  formation 
of  entirely  new  entities  to  take  the  places  of  those  destroyed. 

The  Phenomenon  of  Stimulation. — Every  cell,  whether 
leading  an  independent  existence  as  an  organism  or  combined 
with  others  to  form  the  tissues  of  the  higher  plants  or  animals, 
is  composed  of  certain  substances  which  are  made  to  interact 
and  to  liberate  energy.  The  question  may  now  be  asked 
whether  these  processes  are  perfectly  spontaneous  or  whether 
they  are  set  in  motion  by  some  outside  force.  Let  us  observe 
for  a  moment  an  amoeba  as  it  moves  from  place  to  place 
seeking  its  sustenance.  It  sends  out  its  pseudopods  in  one 
particular  direction,  meanwhile  retracting  its  protoplasm 
elsewhere,  until  it  comes  in  contact  with  a  nutritive  particle, 
such  as  a  diatom.  This  it  gradually  surrounds  until  it 
assumes  a  position  in  its  interior  near  its  posterior  pole. 
In  this  way  a  food  vacuole  is  formed  which  consists  of  the 


GENERAL    CONDITIONS    OF   LIFE  41 

diatom  and  a  small  quantity  of  water.  The  amoeba  con- 
tinues on  its  way,  seeking  other  prey.  Meanwhile  the  en- 
gulfed organism  is  slowly  dissolved.  If  it  contains  coloring 
matter,  this  is  changed  to  a  brown  color  in  consequence  of 
oxidation.  Nothing  is  left  of  it  after  a  time  excepting  the 
pieces  of  its  calcareous  shell,  and  these  are  finally  deposited 
in  the  wake  of  the  amoeba.  Obviously,  the  protoplasm  of 
this  free-living  cell  possesses  the  power  of  converting  the 
substance  of  its  prey  into  a  soluble  and  diffusible  form,  so 
that  it  may  be  embodied  in  its  own  mass. 

In  what  particular,  we  may  now  ask,  does  the  behavior  of 
the  higher  animal  differ  from  that  of  the  amoaba?  The 
answer  no  doubt  will  be  that  the  principle  involved  is  abso- 
lutely the  same,  although  certain  differences  are  apparent 
which  are  not  of  fundamental  importance.  In  whatever 
form  living  matter  may  appear,  it  is  constantly  subjected  to 
outside  influences  which  affect  it  in  a  perfectly  definite 
manner.  In  consequence  of  these  influences,  it  must  execute 
certain  reactions,  because  only  when  its  behavior  is  made  to 
conform  precisely  to  the  forces  in  the  universe  can  it  continue 
to  manifest  its  processes  of  life  efficiently^ 

This  point  may  well  be  illustrated  with  the  help  of  a  con- 
crete example,  such  as  is  presented  by  the  activity  of  the 
heart.  It  is  a  remarkable  fact  that  this  organ  will  continue 
to  contract  after  it  has  been  removed  from  the  body  and  has 
been  placed  in  a  dish  under  proper  conditions  of  moisture 
and  temperature.  Apparently,  its  action  is  quite  sponta- 
neous, because  it  seems  to  take  place  without  any  tangible 
external  cause.  Such  movements,  instituted  in  response 
to  an  invisible  stimulus,  are  characterized  as  automatic. 
In  reality,  however,  this  "  spontaneity "  is  the  result  of  the 
action  of  stimuli  in  the  form  of  certain  salts  which  cause  the 
contractile  elements  of  the  heart  muscle  to  undergo  very 
characteristic  reactions.  What  is  true  of  the  heart  is  also 
true  of  other  tissues  and  organs.  Consequently,  it  may 
be  concluded  that  life  is  possible  only  in  the  presence  of 
definite  exciting  agents  or  stimuli  which  cause  living  matter 
to  manifest  those  responses  for  which  it  is  peculiarly  fitted 
by  virtue  of  its  structure  and  composition. 


42  GENERAL    PHYSIOLOGY 

Manifestations  of  Energy. — Living  matter  is  found  in  the 
air  as  well  as  in  the  water.  Both  media  are  teeming  with 
different  forms  of  energy  which  in  general  may  be  classified 
as  vibratory  and  chemical,  and  embrace  the  following 
impacts : 

(A)  Vibratory  Energy. — (a)  Mechanical  impacts  of  differ- 
ent quality  and  frequency. 

(6)  Vibrations  in  material  media,  such  as  air  and  water. 
They  are  slow  and  give  rise  to  the  sensation  of  sound.  Spe- 
cial sense-organs  are  set  aside  for  their  reception,  for  example, 
the  organ  of  Corti  in  the  internal  ear  of  the  mammals. 
The  human  ear  is  capable  of  receiving  vibrations  in  air 
varying  between  30  and  30,000  in  a  second.  The  range  of 
the  well-trained  ear  is  even  greater  than  this,  namely,  50,000 
in  a  second. 

(c)  Vibrations  in  immaterial  media,  such  as  ether  which 
constitutes  a  peculiar  medium  occupying  space  together 
with  the  air.  The  vibrations  in  ether  vary  between  3000 
and  6,000,000,000  in  a  second  and  give  rise  to  the  so-called 
electrical  waves,  heat-rays,  light-rays,  Roentgen-rays,  and 
others. 

(B)  Chemical  Energy. — Chemical  impacts  are  caused  by  a 
large  number  of  substances.     Upon  the  lower  forms  their 
action  is  direct,  whereas  the  higher  animals  receive  them,  as 
a  rule,  through  special  organs,  such  as.  the  taste-buds  and 
olfactory  cells.     The  former  mediate  the  sense  of  taste,  and 
the  latter,   the  sense  of  smell.     A  special  chemical  sense 
evoked  by  certain  end-organs  of  the  skin  is  present  in  many 
organisms.     Since    osmotic    changes    in    the    surrounding 
medium    are   usually   associated   with    chemical   reactions, 
they  do  not  merit  separate  enumeration. 

Classification  of  Stimuli. — It  should  be  clearly  understood 
that  phenomena  of  stimulation  result  only  when  the  force 
or  character  of  the  impacts  acting  upon  living  matter  is 
suddenly  altered.  Accordingly,  a  stimulus  may  be  defined 
as  any  extraordinary  change  in  the  environment  in  which 
the  organism  is  living.  We  have  seen  that  the  energies  in 
space  are  very  diversified,  and  that  any  particular  plant 
or  animal  may  be  more  receptive  toward  one  impact  than 


GENERAL    CONDITIONS   OF   LIFE  43 

another.  In  fact,  some  of  these  energies  may  fail  altogether 
in  evoking  a  reaction  in  certain  types  of  organisms,  because 
the  latter  have  no  means  of  receiving  them.  Thus,  a  sound- 
wave is  quite  unable  to  produce  a  typical  effect  in  amceba 
and  allied  organisms  unless  possibly  as  a  simple  mechanical 
impact  after  it  has  been  transferred  into  vibrations  of  water. 


FIG.  7. — Diatom    liberating    oxygen    attracts    numerous    bacteria. 

(Verworn.) 

Every  organism  is  constantly  subjected  to  particular  types 
of  stimuli,  and  only  in  the  presence  of  these  can  it  fully 
unfold  its  life  processes. 

The  character  of  the  stimuli  to  which  living  matter  may  be 
exposed  is  very  diversified.  In  spite  of  this  fact,  however, 
it  is  possible  to  arrange  them  in  the  following  order: 

(a)  Mechanical. — In  this  group  belong  such  stimuli  as 
touch,  pressure,  stroking,  pulling,  the  forces  of  gravitation, 
adhesion  and  cohesion. 


44  GENERAL   PHYSIOLOGY 

(6)  Chemical. — Various  substances  possessing  either  a 
favorable  or  an  unfavorable  influence  upon  living  matter, 
belong  in  this  group  of  stimuli.  In  addition,  mention  should 
be  made  of  those  excitants  which  are  the  direct  result  of  osmo- 
tic interchanges. 

(c)  Thermal. — These  stimuli  originate  in  changes  in  the 
temperature  of  the  medium. 

(d)  Photic. — Light  is  a  very  potent  and  valuable  stimulus. 
Besides  the  ordinary  rays,  living  matter  may  also  be  sub- 
jected to  particular  types  of  ethereal  vibrations,  such  as 
produce  the  Hertzian  heat  rays  or  the  Roentgen  rays. 

(e)  Electrical. — Phenomena  of  stimulation  also  result  when 
living  matter  is  exposed  to  the  electrical  current.     Magnetic 
stimuli  do  not  merit  special  enumeration,  because  it  is  doubt- 
ful whether  they  are  able  to  influence  living  matter  directly. 

The  Strength  and  Duration  of  Stimulation. — In  rating 
the  effect  of  a  stimulus  attention  should  be  paid  to  its  quality 
as  well  as  to  its  strength,  the  latter  term  serving  as  an  expres- 
sion of  its  intensity,  duration,  and  frequency.  Living  matter 
exhibits  the  most  efficient  reactions  when  the  stimuli  acting 
upon  it  possess  a  moderate  duration  and  strength.  At  this 
time  optimum  conditions  are  said  to  prevail  towards  which 
it  reacts  in  an  optimum  manner.  But,  stimuli  may  also 
increase  in  intensity,  becoming  first  maximal  and  later  on 
supramaximal  in  character.  Toward  the  former,  living 
substance  responds  by  increasing  the  amplitude  of  its  reac- 
tions, but,  naturally,  a  maximal  activity  cannot  be  endured 
for  any  length  of  time  and  must  finally  prove  injurious  to  it. 
Supramaximal  stimuli  are  destructive  from  the  start  and 
quickly  result  in  the  death  of  living  matter. 

Lastly,  the  intensity  of  the  stimuli  may  be  so  slight  that 
they  fail  to  excite  living  matter  in  a  proper  manner.  These 
minimal  stimuli  evoke  minimal  responses.  On  reducing  the 
strength  of  the  stimuli  still  further,  a  point  will  eventually 
be  reached  when  the  stimulus  just  barely  produces  a  reaction. 
This  is  the  threshold  value  of  the  stimulation  at  which  the 
slightest  possible  reaction  is  obtained.  Below  this  point, 
the  stimuli  must  fail  to  excite  living  substance  in  an  apprecia- 
ble manner.  To  be  sure,  these  subminimal  stimuli  may 


GENERAL   CONDITIONS   OF   LIFE  45 

produce  certain  chemico-physical  reactions  in  protoplasm, 
but  their  strength  is  not  sufficient  to  induce  a  visible  response. 
Obviously,  life  is  impossible  under  these  circumstances. 

It  is  to  be  noted,  therefore,  that  death  ensues  whenever  the 
realm  of  stimulation  is  extended  beyond  its  minimal  or 
maximal  limits.  It  should  be  remembered,  however,  that 
optimum  conditions  are  not  always  found  strictly  midway 
between  these  two  extremes.  Protoplasm  differs  consider- 
ably in  its  constitution,  so  that  stimuli  possessing  an  optimum 
strength  for  one  type,  may  not  be  equally  effective  for  an- 

snu         W  o  m        sm 

*     I  <- 


FIG.  8. — Intensity  of  stimulation.  L,  life;  Z>,  death;  SMi,  subminimal; 
Mi,  minimal;  0,  optimum;  M,  maximal;  SM,  supramaximal  stimuli; 
T,  threshold. 

other.  Thus,  the  temperature  of  the  Arctic  Ocean  may  be 
quite  suitable  for  certain  organisms,  but  very  destructive 
to  others. 

Living  matter  also  possesses  the  power  of  adapting  itself  to 
stimuli.  Thus,  a  certain  influence  which  at  first  produces  a 
considerable  response,  may  in  the  course  of  time  become 
quite  ineffective.  This  state  of  adaptation  presents  certain 
similarities  to  the  state  of  refraction,  although  the  latter 
possesses  an  entirely  different  cause.  We  have  seen  that 
every  activity  of  protoplasm  is  associated  with  a  certain  loss 
of  material,  and  that  the  substances  destroyed  must  first  be 
rebuilt  before  another  response  can  be  given.  Consequently, 
too  rapid  a  succession  of  stimuli  must  be  connected  with 
certain  dangers  to  function,  because  it  does  not  allow 
the  protoplasm  sufficient  time  in  which  to  acquire  new 
material  in  preparation  for  the  succeeding  reaction.  A  point 
will  then  be  reached  when  the  succeeding  stimuli  must  fail  to 
evoke  responses.  The  interval  which  protoplasm  absolutely 
requires  for  its  anabolic  processes  and  during  which  it 
remains,  so  to  speak,  impermeable  to  stimuli,  is  designated 
as  the  refractory  period. 


SECTION  II 
MUSCLE  AND  NERVE 

CHAPTER  IV 
MOTION 

The  Arrangement  of  the  Subject-matter. — Physiology 
deals  with  the  functions  of  the  organism.  It  informs  us 
regarding  the  uses  to  which  the  different  parts  of  the  latter 
are  put  in  order  to  obtain  a  co-ordinated  living  whole.  But, 
even  the  lowest  forms  allow  us  to  infer  that  their  behavior  is 
the  product  of  several  functions,  although  the  simpler 
the  living  entity,  the  more  rudimentary  are  the  activities  of 
its  different  component  parts.  Thus,  it  is  a  relatively 
simple  matter  to  study  the  movements  of  the  amoeba,  para- 
moecium  and  allied  organisms,  but  very  difficult  to  analyze 
their  methods  of  respiration,  metabolism,  and  excretion. 
We  know  that  these  functions  must  be  present,  because 
without  them  they  could  not  exist.  This  division  of  labor 
is  more  apparent  in  the  higher  forms,  in  which  we  are  able  to 
recognize  special  groups  of  organs,  each  fulfilling  a  definite 
function,  and  all  combined  imparting  to  the  organism  as  a 
whole  a  definite  general  behavior.  One  complex  of  organs 
attends  to  motion  and  locomotion,  another  to  respiration, 
another  to  the  assimilation  and  dissimilation  of  the  food, 
another  to  excretion,  another  to  the  distribution  of  the 
assimilated  material,  another  to  the  co-ordination  of  all 
these  functions,  and  still  another  to  reproduction  and  the 
propagation  of  the  species. 

This  division  of  function  makes  it  possible  for  us  to  ap- 
proach the  subject  of  physiology  in  an  analytical  manner. 
Somewhat  in  accordance  with  the  methods  practiced  in  the 

46 


MOTION  47 

dissecting  room,  the  gross  function  of  an  organism  may  be 
separated  into  its  minor  contributory  functions,  and  having 
obtained  these  single  parts,  we  may  again  fit  them  into  one 
another  in  an  endeavor  to  observe  the  working  of  the  whole. 
The  most  concrete  subjects  of  physiology  are  those  which  deal 
with  motion  and  locomotion,  the  circulation  of  the  blood, 
the  interchange  of  the  gases,  the  assimilation  and  dissimila- 
tion of  the  food,  and  the  nervous  control  of  these  functions. 
While  these  subjects  permit  of  a  different  arrangement,  it  is 
advantageous  to  place  motion  first,  because  it  is  most  easily 
understood  and  introduces  the  student  to  physiology,  so  to 
speak,  through  the  channel  of  least  resistance. 

Different  Types  of  Motion. — By  virtue  of  its  property  of 
irritability,  protoplasm  is  able  to  receive  stimuli  and  to 
liberate  in  consequence  of  them  certain  forms  of  energy,  such 
as  mechanical  work,  heat,  and  electricity.  The  mechan- 
ical form  of  energy  is  based  upon  motion  and  locomotion. 
These  manifestations  of  protoplasmic  activity  present  them- 
selves either  as  passive  or  active  movements,  the  very 
diversified  character  of  which  is  due  to  differences  in  the 
shape  and  size  of  the  organs  producing  them.  A  movement 
may  follow  any  one  of  the  changes  enumerated  in  the 
succeeding  table: 

A. — Passive,  due  to   the  dynamic   condition  of  matter, 
because  its  molecules  are  always  moving. 

Motion  Swelling  of  the  cell  wall 

Changes  in  the  cell-turgor 

Changes  in  the  specific  gravity 

Secretion 

Growth  f  amoeboid 

Contraction  and  Expansion  i  ciliary 

{  muscular 

A  passive  movement  follows  the  impact  of  an  outside  force 
upon  a  movable  object.  If  we  observe  the  circulation  of  the 
blood  in  the  web  of  the  frog,  the  different  cellular  elements  of 
this  fluid  will  be  seen  to  traverse  the  vessels  with  a  definite 
velocity;  however,  their  progress  is  not  due  to  an  activity 
of  their  own  but  to  an  outside  force  resident  in  the  pumping 
action  of  the  heart.  Active  movements  result,  for  example, 


48  MUSCLE    AND    NERVE 

in  consequence  of  the  absorption  of  water  by  an  expansible 
body.  If  a  dry,  wedge-shaped  piece  of  wood  is  placed 
between  two  stones  and  is  allowed  to  imbibe  water,  it 
gradually  increases  in  volume  and  presently  forces  the 
stones  apart.  This  type  of  motion  is  most  plastically 
portrayed  by  the'  so-called  resurrection  plants  of  desert 
regions.  When  dry,  these  plants  possess  the  appearance  of 
crumbled-up  filaments  of  leaf,  while  in  a  moist  atmosphere 
they  unfold  their  parts  and  assume  more  definite  shape. 


FIG.  9. — Sensitive    plant    (Mimosa    pudica).     A.    Resting    position.     B. 
Stimulated.      (Verworn  after  Detmer.) 

A  very  interesting  movement  is  brought  about  in  certain 
plants  by  changes  in  the  turgor  or  tension  of  the  cells.  Owing 
to  rapid  osmotic  interchanges  and  the  contraction  of  the 
primordial  sac,  the  elastic  wall  of  the  cells  is  made  to  fluctu- 
ate. In  the  so-called  sensitive  plants  this  movement  is 
very  sudden  and  may  be  evoked  by  different  stimuli.  Merely 
touching  a  plant  of  this  kind  (mimosa  pudica)  causes  it  to 
close  up  its  leaflets  and  to  droop  its  stems.  A  similar  reaction 
occurs  when  the  intensity  of  the  light  is  lessened.  In  this 
group  also  belong  the  common  bladderwort,  Venus'  flytrap, 
and  certain  pitcher  plants.  "The  apical  portions  of  the  leaves 
of  these  plants  are  modified  to  form  pouches  which  are  beset 
with  hair-like  projections.  These  hairs  are  directed  down- 
ward and  are  sensitive  to  stimuli.  The  slightest  touch  causes 


MOTION 


49 


the  leaf  to  close  along  the  midrib,  thereby  entrapping  its 
prey.  The  leaf  contains  many  glands  which  then  pour  forth 
a  secretion  capable  of  digesting  diverse  nitrogenous  materials 
by  virtue  of  a  particular  proteid-splitting  enzyme. 

Movements  by  changes  in  the  specific  gravity  of  the  organism 
are  most  convincingly  betrayed  by  the  radiolaria.  Ordinarily 
heavier  than  water,  these  cells  live  upon  the  bottom  of  stag- 
nant pools.  They  rise  to  higher  levels  in  consequence  of  the 


FIG.   10. — Radiolaria  cell  showing  central  nucleus  and  zone  of  vacuoles 
surrounding  it.      (Verworn.) 

deposition  of  bubbles  of  metabolic  carbon  dioxid  among  their 
radiate  prolongations.  Their  downward  movement  begins 
immediately  after  these  gaseous  formations  have  been  broken 
up  in  the  agitated  layers  near  the  surface  of  the  water. 

Movements  by  secretion  may  be  studied  in  algae  and  oscil- 
larise.  These  organisms  project  a  sticky  material  from  their 
bodies  which  aids  them  in  gliding  from  place  to  place.  Move- 
ments by  growth  are  very  common  but  usually  very  slow. 
Much  more  rapid  movements  of  this  kind  may  be  elicited 
from  the  seeds  and  fruits  of  many  plants.  Thus,  it  is  a 
matter  of  common  experience  that  the  touching  of  certain 
seeds  causes  their  capsular  investments  to  burst  in  conse- 
quence of  the  high  degree  of  tension  stored  up  within  their 
substance. 


50  MUSCLE   AND    NEEVE 

Among  animals  the  most  striking  movement  is  accom- 
plished by  means  of  protoplasmic  contraction  and  expansion. 
Owing  to  the  presence  of  a  certain  amount  of  contractile 
material  within  their  protoplasm,  they  are  able  to  assume  a 
more  compact  and  rounded  form  when  stimulated.  This 
state  is  followed  sooner  or  later  by  that  of  expansion  or 
relaxation  during  which  the  entire  mass  becomes  flatter  and 
larger  in  size.  Contraction  signifies  stimulation,  whereas 
relaxation  indicates  rest  and  the  absence  of  stimuli.  In 
this  connection,  we  should  recall  to  our  minds  the  mode  of 
progression  of  amceba  and  allied  organisms,  which  exemplifies 
the  so-called  amceboid  movement.  But,  in  many  instances, 
the  main  mass  of  the  organism  remains  rather  immotile, 
while  it  as  a  whole  is  moved  onward  by  certain  accessory 
structures,  such  as  cilia,  flagella,  and  muscles.  Inasmuch  as 
these  contractile  elements  are  present  in  all  the  higher  forms, 
it  seems  advantageous  to  study  them  with  at  least  a  fair 
degree  of  detail. 

Amoeboid  Motion. — The  amceba  represents  the  simplest 
form  of  life,  because  it  consists  of  a  single  bit  of  protoplasm. 
It  does  not  possess  a  fixed  shape,  but  is  constantly  shifting  its 
mass,  thereby  enabling  it  to  send  out  delicate  processes  in  one 
direction  and  retracting  its  substance  elsewhere.  In  doing 
this  it  will  be  noted  that  its  granular-  central  portion  or 
endoplasm  is  moved  less  easily  than  its  outer  hyaline  zone  or 
ectoplasm.  No  definite  part  of  this  cell  is  set  aside  for  the 
reception  of  food,  although  it  usually  moves  about  in  such  a 
way  that  these  nutritive  particles  come  to  lie  in  its  posterior 
portion.  Like  other  cells  it  takes  in  oxygen  and  discharges 
carbon  dioxid,  liberating  in  consequence  of  these  oxidative 
processes  a  definite  amount  of  energy.  How  this  end  is 
accomplished  is  not  known,  because  this  organism  is  alto- 
gether too  small  to  be  able  to  observe  these  changes  directly. 

The  type  of  movement  displayed  by  amceba  is  not  re- 
stricted to  this  particular  form,  but  is  also  exhibited  by  the 
rhizopods,  egg  cells,  pigment  cells,  giant  cells,  and  the 
leukocytes  of  the  higher  animals.  The  leukocytes  make  use 
of  their  power  of  amceboid  motion  in  engulfing  foreign 
material  which  has  entered  the  blood  or  tissues.  This  they 


MOTION 


51 


digest  and  render  inert,  thereby  protecting  the  body  against 
injury 

Ciliary  Motion. — Many  free-living  cells  move  by  means  of 
one  or  two  long  threads  of  protoplasm  protruding  from  one 
pole  of  their  body.  These  filaments  are  known  as  flagella. 
When  contracting  they  swing  into  a  position  next  to  the 
sides  of  the  cell-body,  thereby  forcing  the  latter  forward. 
A  very  similar  motile  mechanism  is  furnished  by  the  cilia 


FIG.   11. — An  amoeba,  showing  different  stages  of  movement.     (Verworn.) 

which  present  themselves  as  short  hair-like  processes  pro- 
jecting in  great  numbers  from  the  surface  of  the  cell.  The 
action  of  these  protoplasmic  filaments  may  be  studied  with 
advantage  in  certain  protozoa,  such  as  the  paramcecium  or 
slipper  animalcule.  When  placed  under  the  ocular  of  a 
microscope,  this  organism  will  be  seen  to  possess  an  elongated, 
oval  outline,  one  of  its  poles  being  more  pointed  than  the 
other.  Its  surface  is  everywhere  beset  with  delicate  hair- 
like  processes  which  contract  at  regular  intervals  and  in  a 
definite  direction,  thereby  lashing  the  water  like  tiny  oars 
and  causing  the  cell  as  a  whole  to  progress  in  a  direction 
opposite  to  their  stroke.  This  unicellular  organism  possesses 


52 


MUSCLE    AND    NERVE 


a  higher  organization  than  amoeba,  because  the  particles  of 
food  reach  its  interior  through  a  funnel-shaped  depression 
in  its  surface  and  appear  later  on  as  food-vacuoles  inside 
its  protoplasm.  In  addition,  this  organism  exhibits  con- 
tractile vacuoles  which  pulsate  at  regular  intervals  and 
appear  to  play  the  part  of  excretory  and 
circulatory  organs. 

Cilia  are  widely  distributed  throughout 
the  animal  kingdom.  In  the  higher  forms 
they  appear  upon  the  free  surfaces  of  the 
cells  lining  the  respiratory  and  digestive 
tracts,  as  well  as  upon  those  lining  the 
uro-genital  passage.  In  man,  we  find  them 
upon  the  mucous  membrane  of  the  nasal 
passage,  lacrimal  duct  and  sac,  Eustachian 
tube  and  tympanic  cavity,  upper  portion  of 
the  pharynx  and  larynx  with  the  exception 
of  the  vocal  cords,  trachea  and  bronchi, 
Fallopian  tube,  vagina,  central  canal  of  the 
spinal  cord,  and  the  ventricles  of  the  cere- 
brum. During  embryonal  life  they  are 
also  present  in  the  mouth,  esophagus,  and 
stomach. 

The  cilia  found  in  the  trachea  of  man 
measure  only  0.003  to  0.005  mm.  in  length 
FIG.  12. — Para-  and  0.0003  mm.  in  thickness.  Much  better 
moecium,  a  ciliated  preparations  may  be  obtained  from  the 
mucous  lining  of  the  mouth  of  the  frog  or 
from  that  of  the  gill-plates  of  the  clam.  If  a  piece  of  one 
of  these  membranes  is  placed  under  the  ocular  of  a  micro- 
scope, so  that  its  edge  is  brought  into  view,  it  will  be 
noted  first  of  all  that  the  water  next  to  the  surface  is 
moved  in  the  form  of  a  definite  stream.  The  small  parti- 
cles suspended  therein  proceed  in  the  same  direction,  being 
abruptly  projected  onward  whenever  they  come  in  con- 
tact with  the  lining  cells.  On  closer  observation  we  may 
then  make  out  the  individual  cilia,  forcibly  striking  in  a 
particular  direction  and  again  bending  back  into  their  original 
positions. .  Inasmuch  as  each  cilium  is  firmly  anchored  with 


MOTION 


53 


its  basal  portion  in  the  marginal  zone  of  the  lining  cell,  it 
executes  a  movement  in  a  definite  plane,  becoming  sickle- 
shaped  while  contracting.  This  movement  constitutes  its 
effective  stroke.  When  relaxing  the  cilium  moves  in  the 
reverse  direction  until  it  has  again  assumed  a  position  almost 
parallel  to  the  surface. 

Since  a  ciliated  infusorium  is  equipped  with  many  thou- 
sands of  these  cilia,  and  even  a  single  lining  cell  may  be  beset 


FIG.   13.— Ciliated  cells, 
from    mucosa 


A,  from  a  liver  duct  of  the  garden  snail;  B, 
of    frog.      (After    M.    Haidenhain.} 


with  several  hundreds  of  these  contractile  projections,  the 
question  may  be  asked  how  these  structures  can  avoid 
striking  against  one  another.  It  is  to  be  noted  that  the 
different  rows  of  cilia  contract  successively,  those  in  the  first 
row  always  being  in  a  more  complete  state  of  contraction 
than  those  in  the  second,  and  so  on  until  the  last  line  has 
been  reached.  In  this  way,  the  entire  ciliated  area  is  divided 
functionally  into  several  smaller  ones,  simulating  somewhat 
the  effect  produced  by  a  gust  of  wind  as  it  strikes  a  field  of 
wheat. 

The  question  may  also  be  asked  whether  the  action  of 
these  different  rows  of  cilia  is  co-ordinated  by  nerve  fibers. 
Inasmuch  as  the  existence  of  such  fibers  has  not  been  proven 
and  seems,  moreover,  very  unlikely,  we  must  conclude  that 


54 


MUSCLE    AND    NERVE 


the  cilia  are  able  to  influence  one  another  by  simple  proto- 
plasmic continuity.  It  is  also  of  interest  to  note  that  a 
reversal  of  the  effective  stroke  of  the  cilia  has  been  observed 
in  certain  organisms.  In  the  sea  anemone,  for  example,  the 
cilia  around  the  edge  of  the  mouth  usually  beat  from  without 
inward,  thereby  directing  a  steady  stream  of  water  into  the 
body-cavity.  If,  however,  a  non-nutritive  substance,  such 
as  a  granule  of  sand,  is  placed  upon  the  oral  margin,  the  cilia 
reverse  their  beat  in  an  endeavor  to  expel  this  foreign  particle. 


FIG.  14. — Movement  of  a  single  cilium.     A,  Progressive  in  direction  of 
arrow;  B,  Regressive.     (After  Verworn.) 

Since  the  contraction  of  each  cilium  is  executed  somewhat 
after  the  manner  of  a  whip  when  lashed,  it  must  be  evident 
that  any  particle  coming  in  contact  with  it  must  be  projected 
in  the  direction  of  its  effective  stroke.  This  fact  may  be 
illustrated  by  placing  a  few  bits  of  cork  upon  the  exposed 
mucous  lining  of  the  frog's  mouth.  These  particles  will  then 
be  seen  to  move  in  the  direction  of  the  esophagus  and,  if  the 
latter  has  been  laid  open,  into  the  stomach.  In  the  adult 
mammal,  the  lining  membrane  of  the  digestive  tract  is  non- 
ciliated  throughout.  Cilia,  however,  are  found  in  the 
respiratory  passage,  where  they  beat  towards  the  outside. 
Their  function  is  to  move  the  particles  of  dust  into  the  phar- 
ynx, whence  they  are  flushed  into  the  stomach  by  the  saliva. 
It  is  true,  however,  that  a  certain  proportion  of  dust  always 
gets  beyond  these  ciliated  regions  into  the  finer  bronchioles 


MOTION 


55 


and  alveoli  of  the  lungs.  Thus,  the  domestic  animals  and 
inhabitants  of  the  cities  commonly  present  lungs  consider- 
ably stained  with  coal  dust.  It  is  true,  however,  that  a 
much  greater  amount  of  this  foreign  material  would  be  able 
to  enter  if  these  tubules  were  not  ciliated.  Particularly 
heavy  depositions  of  dust  are  frequently  found  in  the  lungs 
of  coal  miners  and  marble  cutters.  Nature  eventually 


FIG.  15. — The  sea  anemone. 

endeavors  to  dislodge  them  by  a  catarrhal  inflammatory 
reaction  which  may  at  times  assume  the  general  character 
of  tuberculosis. 

Muscular  Movements. — Besides  the  protozoa  or  one- 
celled  entities,  the  animal  world  also  embraces  many-celled 
organisms  or  metazoa.  The  latter  group  presents  a  much 
higher  type  of  motion  than  the  former.  This  change  is  de- 
pendent upon  the  development  of  a  more  complex  tissue  which 
is  designated  as  muscle  tissue  and  is  made  up  of  cells  possess- 


56 


MUSCLE    AND    NERVE 


ing  an  extraordinary  contractile  power.  This  statement,  how- 
ever, is  not  meant  to  imply  that  amoeboid  and  ciliary  motions 
are  absent  in  the  higher  forms.  Neither  should  it  be  inter- 
preted as  suggesting  that  muscle  cells  are  not  found  in  the 
lower  forms,  because  several  infusoria  exhibit  in  certain 


FIG.   16. — Vorticella  in  the  resting  position  (A)  and  position  of  stimulation 
(B).     The  myoids  are  indicated  in  red. 

parts  of  their  protoplasm  long  fibrillar  structures  which 
shorten  on  stimulation,  thereby  causing  the  length  of  the 
entire  organism  to  be  instantaneously  diminished.  But, 
these  fibrils  are  not  identical  with  the  muscle  cells  of  the 
mammals,  although  it  must  be  admitted  that  they  present 
certain  similarities  in  structure  as  well  as  in  action.  They 
are  usually  designated  as  myoids. 


MOTION  57 

In  the  organism  known  as  stentor,  these  contractile  elements 
appear  as  single  filaments  below  the  envelope  of  its  trumpet- 
shaped  body,  whereas  in  vorticella  they  are  united  into  a 
bundle  which  occupies  the  stalk  upon  which  the  bell-shaped 
upper  portion  of  its  body  is  situated.  During  moments  of 
non-stimulation  either  one  of  the  organisms  just  mentioned 
extends  its  oral  pole  far  into  the  water,  permitting  the  cilia 
upon  the  margin  of  its  mouth-cavity  to  divert  particles  of 
food  into  its  interior.  When  stimulated,  its  head-end  is 
swiftly  retracted  toward  its  basal  portion  which  serves  as  the 
point  of  attachment  for  the  organism  as  a  whole. 

Broadly  speaking,  it  may  be  stated  that  these  myoids  are 
the  precursors  of  the  muscle  cells  of  the  higher  animal.  They 
pass  through  several  evolutionary  stages,  giving  rise  first  of 
all  to  the  so-called  smooth  muscle  cell  and  eventually  to  the 
striated  muscle  cell.  A  type  somewhat  different  from  these 
is  the  cell  of  the  cardiac  muscle  tissue.  When  observing  a 
higher  animal,  we  note  that  it  moves  from  place  to  place  by 
means  of  large  masses  of  muscle  tissue  which  are  attached  to 
its'  bony  framework.  The  constituents  of  the  latter  are 
employed  as  levers  to  increase  their  power.  This  type  of 
muscle  tissue  is  characterized  as  striated,  because  its  cellular 
components  present  distinct  cross-striations  when  studied 
with  the  aid  of  the  microscope.  It  is  also  designated  as 
skeletal  muscle  tissue  and  as  voluntary  muscle  tissue,  because 
it  is  attached  to  the  bony  framework  of  the  body,  and  is 
under  the  direct  control  of  the  will.  This  statement  implies 
that  the  contraction  of  every  striated  muscle  is  instigated  by 
impulses  which  are  conducted  to  it  from  nerve  cells  situated 
in  a  particular  region  of  the  central  nervous  system. 

Besides  this  general  movement  of  locomotion,  an  animal  of 
this  kind  also  exhibits  certain  movements  of  its  internal 
organs  or  viscera.  Its  stomach  and  intestines  contract 
upon  the  ingested  material,  reducing  it  in  a  mechanical  way. 
Its  bloodvessels  constrict,  thereby  varying  the  size  of  its 
vascular  channels.  Its  urinary  receptacle  contracts  at 
definite  intervals  in  an  attempt  to  expel  its  contents.  These 
internal  movements  are  accomplished  by  means  of  smooth 
muscle  cells  which  are  embedded  in  the  connective  tissue 


58  MUSCLE   AND    NERVE 

framework  of  these  organs.  They  are  characterized  as 
smooth  or  plain,  because  they  do  not  present  a  striated 
appearance  in  microscopic  vision,  and  are,  therefore,  most 
closely  related  to  the  myoids.  In  other  words,  smooth 
muscle  is  a  more  primitive  type  than  the  striated  variety. 
Inasmuch  as  these  cells  are  found  chiefly  in  the  viscera,  they 
form  what  might  be  termed  the  visceral  muscle  tissue.  More- 
over, since  the  movements  observed  in  these  internal  organs 
are  not  ui}der  the  control  of  the  will,  this  tissue  may  also  be 
characterized  as  involutary  muscle  tissue.  This  statement 
implies  that  the  nervous  impulses  which  activate  it,  originate 
in  nerve  centers  which  are  not  dominated  by  volition. 

The  muscle  tissue  of  the  heart  presents  a  structure  some- 
what different  from  that  of  the  preceding  ones.  In  addition, 
it  may  easily  be  noted  that  its  activity  is  not  the  result  of 
external  stimuli,  although  it  is  a  well  known  fact  that  in- 
ternal stimuli  of  some  sort  are  at  work  to  make  it  contract. 
A  tissue  which  continues  to  react  rhythmically  without  any 
apparent  external  cause,  is  designated  as  an  automatic 
tissue. 


CHAPTER  V 


,i 


THE    STRUCTURE   AND    GENERAL   BEHAVIOR    OF 
MUSCLE  TISSUE 

Smooth  Muscle  Tissue. — If  we  examine  an  organ  such  as 
the  stomach,  intestine,  or  ureter,  we  find 
that  it  is  made  up  of  several  of  the  four 
principal  tissues.  Its  supporting  web  con- 
sists of  connective  tissue  cells,  in  which  are 
embedded  varying  numbers  of  smooth 
muscle  cells.  The  latter  are  usually  ar- 
ranged transversely  as  well  as  longitudinally 
to  the  long  axis  of  the  cavity  of  the  organ. 
The  circular  layer  is  situated  as  a  rule  next 
to  the  epithelial  lining  or  mucosa,  while  the 
longitudinal  one  lies  close  to  the  external 
membranous  covering  of  the  organ.  Be- 
sides, an  organ  of  this  kind  always  contains 
nerve  fibers  and  nerve  cells  which  regulate 
its  function,  as  well  as  bloodvessels  and 
lymphatics  subserving  its  metabolic  require- 
ments. 

Smooth  muscle  tissue,  therefore,  consists 
essentially  of  a  sheet  of  connective  tissue 
moulded  to  form  a  tube  and  equipped  with 
varying  numbers  of  smooth  muscle  cells. 
The  internal  surface  of  this  tube  is  lined 
with  epithelium,  while  its  external  surface 
is  covered  with  a  layer  of  thin   plate-like    smooth  muscle 
cells.     Inasmuch  as  large  numbers  of  these    cells,  teased  apart 
smooth  muscle  cells  are  arranged  circularly    and,  showjn.g  long 

*      .  J     oval    nuclei    sur- 

around  the  lumen  of  the  tube,  their  contrac-    rounded   by  un- 
tion  must  give  rise  to  a  diminution  in  the    differentiated 
size  of  the  latter.     This  particular  point    Prot°Plasm- 
may  well  be  illustrated  by  a  brief  study  of  the  character 

59 


FIG.       17.— 


60  MUSCLE    AND    NERVE 

of  the  movements  occurring  along  the  intestinal  canal.  At 
the  height  of  digestion,  waves  of  constriction  are  seen  to 
progress  over  it  which  are  commonly  designated  as  peristaltic 
waves.  Every  one  of  these  waves  consists  of  a  zone  of 
constriction  which  is  preceded  by  a  zone  of  relaxation.  In 
this  way,  the  contents  of  the  canal  are  forced  onward  in  the 
direction  of  least  resistance.  It  is  to  be  noted,  however, 
that  these  movements  are  participated  in  by  both  layers  of 
muscle  cells,  the  circular  one  as  well  as  the  longitudinal. 

The  manner  of  action  of  these  contractile  elements  may 
also  be  observed  in  the  iris,  a  membranous  diaphragm  or  stop 
situated  in  front  of  the  lens  and  between  the  anterior  and 
posterior  chambers  of  the  eye.  It  is  a  matter  of  common 
knowledge  that  the  size  of  its  central  orifice,  the  pupil,  is 
changed  repeatedly  in  accordance  with  the  intensity  of  the 
light.  This  reaction  is  made  possible  by  numerous  smooth 
muscle  cells  which  '  are  arranged  either  circularly  around  the 
pupil  or  radially  to  it.  On  contraction  of  the  former  layer, 
the  margin  of  the  iris  is  moved  inward,  thereby  diminishing 
the  size  of  this  orifice.  A  smaller  bundle  of  light  rays  is  then 
permitted  to  enter  the  interior  of  the  eyeball.  Contrariwise, 
the  contraction  of  the  radial  cells  retracts  the  margin  of  the 
iris,  thereby  increasing  the  size  of  the  central  opening.  At 
this  time  a  larger  number  of  light  rays  is  allowed  to  reach 
the  interior  of  the  eye. 

When  one  of  these  muscle  cells  is  examined  under  the 
microscope,  it  is  found  to  possess  a  spindle-like  shape  and  a 
length  varying  in  different  organs  between  45  and  225^ 

(average:  ~—  of   an  inch)'.     Its  breadth   varies  between  4 
500 

and  7/z  (average:  T^.  of  an  inch).     The  nucleus  occupies 


a   central   position   and   exhibits   a   long-oval   shape.     The 
cytoplasm  presents  a  delicate  longitudinal  striation. 

Striated  Muscle  Tissue.  —  When  examining  a  skeletal 
muscle,  such  as  the  gastrocnemius,  it  will  be  found  that  it  is 
enveloped  by  a  sheath  of  connective  tissue  which  is  called 
the  perimysium.  A  large  number  of  membranous  partitions 
proceed  inward  from  the  inner  surface  of  this  envelope, 


STRUCTURE  AND  BEHAVIOR  OF  MUSCLE  TISSUE 


61 


u 


subdividing  the  space  within  the  perimysium  into  numerous 
smaller  compartments  in  which  the  individual  striated 
muscle  cells  are  situated.  Each  cell  is  surrounded  by  a  wall 
or  sarcolemma,  neighboring  cells  being  -f 

cemented  together  by  a  small  amount  of 
intercellular  substance. 

A  muscle  of  this  kind  presents  three 
principal  parts:  namely,  a  point  of  attach- 
ment, a  body,  and  a  point  of  insertion.  The 
upper  pole  of  such  a  muscle  invariably  con- 
tains fewer  muscle  cells,  and  its  connective 
tissue  is  modified  to  connect  it  firmly  with 
the  bone.  Below  this  point  of  attachment 
the  muscle  cells  greatly  increase  in  number 
at  the  expense  of  the  connective  tissue. 
This  portion  of  the  muscle  is  known  as  its 
body  or  belly.  Below  this  level  the  connec- 
tive tissue  again  increases  in  bulk,  gradually 
displacing  the  muscle  cells.  Finally,  a 
slender  tendon  is  formed  which  is  securely 
fastened  to  the  periosteal  lining  of  a  freely 
movable  bone.  This  constitutes  its  point 
of  insertion.  Everywhere  else  the  muscle 
is  related  to  neighboring  structures  through 
the  medium  of  loose  fascia,  an  extensive 
net- work  of  connective  tissue.  The  blood 
vessels  and  nerves  traverse  its  interior  by 
following  the  partitions  of  connective  tissue. 

Contrary  to  the  smooth  muscle  cells,  the 
striated  cells  are  united  into  compact  masses  de.  A,  Attach- 
which  are  fastened  to  different  parts  of  the  ment;  B,  body, 
skeleton  without  consuming  an  unusual  ha?vlsT T,°Tendo 
amount  of  space.  By  far  the  largest  num-  Achiilis;  I,  inser- 
ber  of  these  muscles  is  concerned  with  the  tion  int°  °alcan- 

f    i  , .  i  .  ,         f.          ,,    .  eum  or  heel-bone. 

process  of  locomotion  which,  after  all,  is  a 
most  important  function,  because  upon  it  depends  practi- 
cally the  only  means  which  the  animal  possesses  to  obtain 
its  food.     It  should  be  remembered,  however,  that  several 
of  the  skeletal  muscles  are  related  to  locomotion  onlv  in  an 


FIG.    18— Gas- 
trocnemius    mus- 


62  MUSCLE    AND    NERVE 

indirect  way;  for  example,  those  of  the  abdominal  wall 
which  are  primarily  intended  to  form  a  flexible  covering 
for  this  cavity  and  to  help  in  keeping  the  body  erect.  Still 
farther  removed  from  locomotion  are  those  which  bring  the 
lower  jaw  against  the  upper,  and  those  initiating  the  act  of 
swallowing.  Even  several  of  the  respiratory  muscles  are 
not  directly  concerned  with  locomotion. 

It  is  also  of  interest  to  note  that  the  general  arrangement  of 
the  cells  is  not  the  same  in  all  muscles,  a  point  which  may 
readily  be  proved  by  a  comparison  of  the  sartorius  and 
gastrocnemius  muscles.  The  former  possesses  a  long  and 
slender  shape,  and  its  components  are  arranged  practically 
parallel  to  one  another.  A  muscle  of  this  kind  is  not  very 
powerful,  but  is  able  to  lift  a  weight  to  a  considerable  height. 
Contrariwise,  the  gastrocnemius  which  is  the  chief  calf 
muscle,  possesses  great  strength,  although  it  is  unable  to 
shorten  very  materially.  This  peculiarity  in  its  behavior 
may  be  ascribed  to  the  fact  that  it  is  built  up  around  a 
strong  core  of  connective  tissue  from  which  the  individual 
muscle  cells  extend  in  an  oblique  direction  downward  and 
outward,  finally  terminating  in  its  thick  outer  investment. 
On  contracting,  these  cells  pull  upon  this  connective  tissue 
capsule  which  in  turn  lifts  the  tendon  of  Achillis  and  extends 
the  foot  upon  the  leg. 

If  a  stained  preparation  of  striated  muscle  tissue  is  placed 
under  the  ocular  of  a  microscope,  it  will  be  noted  immediately 
that  its  individual  cells  are  relatively  long  and  thin.  For 
example,  those  composing  the  sartorius  muscle  of  the  frog 
may  attain  a  length  of  3  to  4  cm.,  but  a  diameter  of  only  1  to 

Wfji  (average:  —  of  an  inch).     For  this  reason,  they  are 

4L/U 

generally  designated  as  fibers.  Their  ends  are  somewhat 
pointed  and  are  joined  with  others  in  series  by  intermediary 
strands  of  connective  tissue.  If  one  of  these  cells  is  observed 
under  the  high  power  of  a  microscope,  it  will  be  seen  to  be 
invested  by  a  wall  or  sarcolemma,  underneath  which  are 
placed  several  nuclei  in  series.  The  interior  of  each  cell  is 
occupied  by  contractile  material  which  exhibits  alternate 
dark  and  light  bands,  similar  to  a  glass  rod  which  has  been 


STRUCTURE   AND   BEHAVIOR   OF   MUSCLE   TISSUE          63 


FIG.  19.— Striated  muscle.  (Slightly  magnified.)  A,  cross  section; 
B,  longitudinal  section;  Sarc.,  sarcolemma;  Bl,  blood  vessels;  E,  endomy- 
sium;  TD,  transverse  disc;  ID,  intermediate  disc;  LD,  Lateral  disc. 


64 


MUSCLE   AND    NERVE 


converted  into  ground  glass  at  regular  intervals.  The  dark 
bands  are  known  as  transverse  discs,  and  the  lighter  ones  as 
lateral  discs.  Each  measures  about  2/x,  in  height,  so  that  a 
segment  of  a  length  of  1  cm.  contains  close  to  10,000  dark 
striae. 

The  theories  which  have  been  put  forth  in  order  to  estab- 
lish a  correlation  between  the  structure  and  the  function 
of  this  cell  are  many,  but  none  may  rightly  be  said  to  have 
given  a  correct  interpretation  of  actual 
conditions.  It  is  apparent  that  a  con- 
tracting muscle  cell  shifts  its  substance 
in  such  a  way  that  its  length  is  con- 
siderably decreased  in  favor  of  its 
breadth.  During  this  phase  the  dark 
bands  become  light,  and  the  light  bands 
dark.  A  rapid  transfer  of  water  from 
'one  disc  into  the  other  is  said  to  be  re- 
sponsible for  this  change.  What  is  true 
of  each  individual  cell,  is  true  of  the 
muscle  as  a  whole.  On  contraction,  it 
increases  its  breadth  at  the  expense  of 
its  length. 

It  should  be  clearly  understood,  how- 
20.— Schema    ever>  that  a  muscle  does  not  change  its 
volume  when  contracting  and  hence,  does 
not   acquire   material   from   without   in 
order  to  bring  this  change  about.     Its 


cle    in    receptacle  is 
stimulated. 


FIG. 

to  show  that  con- 
tracting muscle  does 
no  t  change  its 
volume.  M,  menis- 
cus of  saline  solu- 
tion; s,  electrodes  contraction  merely  consists  of  a  peculiar 
through  which  mus-  re-arrangement  of  its  substance.  This 
point  may  easily  be  proved  by  placing  a 
muscle,  such  as  the  gastrocnemius  of 
the  frog,  in  a  small  receptacle  which  has  been  filled  with 
boiled  saline  solution  and  is  equipped  with  a  capillary  tube. 
The  saline  enters  this  tube  to  a  certain  level,  forming  here  a 
meniscus.  If  the  muscle  is  now  made  to  contract,  the  latter 
remains  perfectly  stationary. 

Cardiac  Muscle  Tissue. — The  wall  of  the  heart  is  composed 
of  a  lining  or  endocardium,  a  layer  of  muscle  tissue  or  myo- 
cardium, and  an  enveloping  membrane  or  epicardium.  The 


STRUCTURE   AND  BEHAVIOR   OF   MUSCLE   TISSUE 


65 


latter  is  reflected  from  the  large  blood  vessels  in  the  form  of 
the  pericardium,  enclosing  a  very  narrow  space  which  is 
filled  with  a  lymph-like  fluid.  Like  in  the  other  contractile 
tissues,  the  myocardium  consists  of  a  framework  of  con- 
nective tissue  in  which  are  embedded  the  individual  rows  of 
cardiac  muscle  cells.  Inasmuch  as  the  heart  possesses  the 
shape  of  a  tube  and  inasmuch  as  the  principal  layer  of  this 
muscle  is  arranged  circularly  around  the  lumen  of  its  cavity, 


FIG.  21.4. — Cardiac  muscle. 


FIG.  21 B. — Single  cardiac  cells. 
Magn.  1000. 


the  contraction  of  these  cells  must  decrease  its  capacity, 
thereby  placing  the  blood  within  under  a  higher  pressure 
than  before. 

If  we  now  place  one  of  the  fibers  or  rows  of  cardiac  muscle 
cells  under  the  high  power  of  a  microscope,  it  will  be  seen 
that  each  component  cell  possesses  the  shape  of  a  short 
cylinder  and  presents  a  rather  square  outline.  Its  rounded 
nucleus  occupies  the  central  region  of  its  cytoplasm.  These 
cells  are  beset  with  blunt  processes  which  are  connected 
with  similar  -projections  from  the  cells  in  the  adjoining 


66  MUSCLE    AND    NERVE 

columns.  This  is  of  the  greatest  functional  importance, 
because  the  wave  of  irritability  causing  this  muscle  to  con- 
tract, is  thereby  enabled  to  skip  from  cell  to  cell  throughout 
the  cardiac  musculature.  Owing,  therefore,  to  its  net-like 
structure,  even  a  slight  stimulus  is  sufficient  to  incite  a 
reaction  in  all  of  its  components.  Contrariwise,  the  cells  of 
striated  and  smooth  muscle  possess  a  certain  independency, 
because  each  element  is  invested  by  sarcolemma  and  inter- 
cellular material.  A  slight  stimulus  applied  to  a  muscle  of 
this  kind  involves  only  a  limited  number  of  contractile 
elements.  For  this  reason,  the  amplitude  of  the  reaction 
of  striated  and  smooth  muscle  must  bear  a  direct  relationship 
to  the  strength  of  the  stimulus,  whereas  cardiac  muscle 
always  reacts  maximally  under  ordinary  conditions  of  exci- 
tation. This  constitutes  the  so-called  all-or-none  law  of 
cardiac  muscle.  It  implies  that  the  contractions  of  this 
tissue  are  always  large  in  amplitude  irrespective  of  the 
strength  of  the  stimulus,  and  that  its  reaction  cannot  be 
graded  accordingly. 


CHAPTER  VI 
THE  MANNER  OF  CONTRACTION  OF  MUSCLE 

Manner  of  Excitation. — Muscle  tissue  is  in  possession  of  a 
certain  amount  of  contractile  substance  which  may  be  acti- 
vated at  intervals  by  stimuli.  Thus,  the  heart  of  a  cold- 
blooded animal  may  be  excised  and  kept  beating  in  a  dish 
for  several  weeks,  provided  it  is  immersed  in  a  nutritive 
solution  of  suitable  temperature.  The  same  results  may  be 
obtained  with  the  hearts  of  warm-blooded  animals.  Con- 
sequently, this  organ  must  embrace  all  the  prerequisites  for 
its  activity,  and  its  connections  with  the  central  nervous 
system  can  only  serve  the  purpose  of  adjusting  its  action  to 
those  of  other  structures.  Likewise,  we  are  able  to  excite 
rhythmic  responses  in  excised  striated  and  smooth  muscle, 
provided  we  expose  these  tissues  to  the  solutions  of  certain 
salts.  Moreover,  it  is  a  matter  of  common  experience  that 
normal  muscles  may  be  activated  by  applying  electrical  or 
mechanical  stimuli  directly  to  the  skin  overlying  them. 

Under  ordinary  circumstances,  however,  both  types  of 
muscle  tissue  are  under  the  control  of  special  nerves  and 
centers,  and  besides,  striated  muscle  is  under  the  influence  of 
volition.  The  nerve  cells  composing  these  centers  generate 
impulses  which  pursue  a  definite  course  outward  to  their 
respective  muscles,  causing  them  to  contract.  As  far  as 
those  skeletal  muscles  are  concerned  which  are  employed  in 
locomotion,  it  is  known  that  their  control  is  effected  through  a 
group  of  large,  pyramidal  nerve  cells  which  are  situated  in 
the  anterior  central  convolution  of  each  cerebral  hemisphere. 
In  accordance  with  its  function,  this  particular  region  of  the 
brain  is  called  the  motor  area.  The  muscles  on  the  left  side 
of  the  body  are  dominated  by  the  cells  in  the  right  half  of  the 
cerebrum,  and  those  on  the  right  side  by  the  cells  in  the  left 
hemisphere.  This  peculiar  innervation  is  brought  about  by 

67 


68 


MUSCLE    AND    NERVE 


the  fact  that  the  nerve  fibers  emerging  from  one 'motor  area, 
cross  over  to  the  other  side  of  the  body  in  the  medulla  and 


FIG.  22. — Diagram  illustrating  the  course  of  the  sensory  and  motor 
fibers  between  the  brain  and  such  parts  as  the  fingers  (D)  and  gastroc- 
nemius  muscle  (G). 

spinal  cord.  For  this  reason,  an  injury  to  one  of  these  motor 
areas  invariably  causes  the  muscles  on  the  opposite  side  to 
become  paralyzed. 


THE   MANNER   OF    CONTRACTION   OF   MUSCLE  69 

It  may  be  said,  therefore,  that  the  skeletal  muscles  are 
activated  under  normal  conditions  in  an  indirect  way  in 
consequence  of  the  influx  of  impulses  generated  in  certain 
nerve  cells.  Whenever  these  impulses  are  unable  to  reach  a 
muscle,  the  latter  remains  inactive.  Such  a  condition  may 
arise  either  in  consequence  of  an  injury  to  the  center  destroy- 
ing its  generating  power,  or  as  a  result  of  a  break  in  the 
conducting  path,  occasioned  in  many  instances  by  nerve 
section.  A  muscle  rendered  inactive  in  this  manner  is  said 
to  be  paralyzed.  Because  of  this  loss  of  stimulation  it 
finally  undergoes  certain  retrogressive  changes  which  betray 
themselves  in  a  diminution  in  its  irritability,  volume,  and 
weight.  The  skeletal  muscles  may  also  be  activated  directly 
by  the  stimulation  of  the  skin  overlying  them  or  by  the 
excitation  of  their  motor  nerves. 

Smooth  muscle  is  also  under  the  control  of  nerve  centers, 
but  these  centers  are  not  dominated  by  volition,  and  fre- 
quently lie  in  the  organ  itself  or  in  its  immediate  vicinity. 
To  be  sure,  these  local  stations  are  connected  with  the  central 
nervous  system  by  special  nerve  fibers,  but  these  paths  are 
present  chiefly  for  the  purpose  of  correlating  the  action  of  one 
organ  with  that  of  another. 

Muscle  Tonus. — When  resting,  a  normal  muscle  is  never 
fully  relaxed,  but  is  retained  in  a  position  of  elastic  tension, 
intermediate  between  complete  relaxation  and  contraction. 
The  condition  of  tonus,  therefore,  simulates  an  incomplete 
relaxation.  Only  a  paralyzed  muscle  ceases  to  be  in  tonus 
and  relaxes  completely.  This  fact  is  important,  because  it 
places  the  muscle  fibers  in  a  position  from  which  they  can 
enter  the  state  of  contraction  more  quickly  than  from  that 
of  complete  extension.  Accordingly,  this  condition  may  be 
said  to  favor  rapidity  of  action  and  to  conserve  muscular 
energy. 

The  cause  of  the  tonicity  of  a  muscle  lies  in  an  influx  of 
impulses  from  those  nerve  cells  which  control  its  action. 
These  impulses  are  discharged  at  the  rate  of  ten  in  every  sec- 
ond and  do  not  evoke  a  visible  reaction.  They  are,  therefore, 
subminimal  in  their  intensity,  causing  the  muscle  substance  to 
retain  a  condition  of  functional  alertness,  ready  at  any 


70  MUSCLE   AND   NERVE 

moment  to  respond  to  impulses  of  supra-threshold  value. 
The  aforesaid  nerve  cells  in  turn  are  forced  to  continue  their 
activity  by  diverse  sensory  impulses.  When  the  latter  cease, 
the  nerve  cells  become  inactive  and  discontinue  to  generate 
those  impulses  upon  which  the  tone  of  muscle  depends. 

Good  posture  depends  upon  the  tonus  of  many  associated 
groups  of  skeletal  muscles.  Thus  we  note  that  the  paralysis 
or^complete  loss  of  tonus  of  a  group  of  muscles  causes  the  part 
moved  by  them  to  become  flaccid  and  to  assume  a  position 
out  of  the  ordinary.  Diminutions  in  the  tonus  of  the 
skeletal  muscles  are  frequently  experienced  in  consequence  of 
mental  and  bodily  fatigue  and  in  the  course  of  many  intoxica- 
tions. Such  organs  as  the  stomach,  intestine,  and  bladder 
undergo  constant  fluctuations  in  tonus.  Hence,  a  stomach 
considerably  enlarged  by  food  need  not  be  more  forcibly 
distended  than  one  practically  empty,  because  its  wall 
accommodates  itself  to  the  varying  amount  of  contents  by 
simply  changing  its  tone.  Accordingly,  the  wall  of  a  highly 
distended  organ  need  not  exert  a  greater  pressure  upon  the 
contents  than  the  one  of  a  partially  filled  organ. 

It  may  rightly  be  concluded  that  a  muscle  out  of  tonus 
cannot  react  as  well  as  one  in  tonus.  This  statement,  how- 
ever, should  not  be  interpreted  as  signifying  that  an  atonic 
muscle  cannot  be  made  to  contract  at  all,  because  an  ab- 
solute functional  uselessness  presupposes  a  severe  disturb- 
ance in  its  metabolic  condition.  Thus  even  a  paralyzed 
muscle  may  be  stimulated  directly  with  positive  results  until 
it  is  no  longer  able  to  acquire  the  necessary  contractile 
material.  Inasmuch  as  the  nutritive  state  and  power  of 
contraction  of  a  muscle  depend  upon  its  activity,  we  usu- 
ally endeavor  to  retain  a  paralyzed  muscle  in  an  at  least 
partial  condition  of  usefulness  by  repeated  massage  and  direct 
electrical  stimulation.  Meanwhile,  possibly,  the  injury  to 
the  nerve  center  or  to  the  conducting  path  may  be  remedied, 
so  that  the  muscle  may  again  come  under  the  influence  of 
normal  impulses  and  regain  its  former  tonus  and  contractile 
strength. 

The  Elasticity  of  Muscle  Tissue. — It  is  a  well-known  fact 
that  a  fresh  rubber  band  may  be  successively  extended  by 


THE    MANNER   OF    CONTRACTION    OF   MUSCLE  71 

weights,  the  degree  of  its  extension  being  each  time  propor- 
tional to  the  weight.  Furthermore,  provided  the  total  load 
is  not  excessive,  the  rubber  band  will  again  recoil  into  its 
former  state  when  the  weights  are  removed.  When  left  in 
the  body,  a  normal  muscle  presents  the  same  tendencies,  i.e.. 
it  resumes  its  previous  shape  and  position  when  deten- 
tioned.  An  excised  muscle,  on  the  other  hand,  is  imper- 
fectly elastic,  and  does  not  fully  recover  its  former  degree 
of  tension  after  it  has  been  stretched  by  weights.  No 
doubt,  this  procedure  has  led  to  a  permanent  displacement  of 


FIG.  23. — Extensibility  and  elasticity.  A,  rubber  band  and  B,  gastroe- 
nemius  muscle  of  frog  successively  loaded  with  10  gram  weights.  The 
second  curve  shows  a  decreasing  extension  for  equal  increments,  hence, 
the  line  joining  the  ends  of  the  ordinates  is  curved. 


its  component  cells.  An  excessive  weight  will  finally  rupture 
the  muscle.  The  breaking  point  in  the  case  of  the  excised 
gastrocnemius  muscle  of  the  frog  lies  near  1000  grams. 
Long  before  this  point  is  reached,  however,  some  of  the 
strands  of  connective  tissue  are  torn  away  from  their  con- 
nections with  the  muscle  fibers.  This  is  the  most  frequent 
cause  of  muscle  strain  and  consequent  stiffness.  The  pain 
felt  in  a  muscle  which  has  been  injured  in  this  way  is  severe 
in  character,  and  may  continue  for  some  time,  indicating 
the  slow  manner  of  repair 'of  a  lesion  of  this  kind.  The 
spraining  and  tearing  of  the  tendons  themselves  usually 
leads  to  extravasations  of  serous  fluid  into  the  adjoining  soft 
parts. 

The  Wave  of  Contraction. — A  short  and  compact  skeletal 
muscle  usually  receives  its  nerve  supply  at  its  upper  pole, 
whereas  a  long  muscle  receives  it  about  midway  between  its 


72  MUSCLE    AND    NERVE 

two  ends.  From  here  the  delicate  fibrils  are  then  distributed 
to  the  individual  cells  in  its  two  poles.  This  arrangement 
possesses  an  important  practical  bearing,  because  if  the 
different  portions  of  a  striated  muscle  actually  contracted  at 
recognizable  intervals,  the  best  mechanical  results  could 
not  be  obtained.  Very  slight  differences  in  their  contraction, 
however,  are  frequently  present,  and  may  be  clearly  demon- 
strated by  means  of  sensitive  measuring  appliances.  Experi- 
mentally, they  may  readily  be  produced  by  the  excitation  of 
a  long  muscle,  such  as  the  sartorius.  When  stimulated  at 
one  of  its  poles,  a  wave  of  contraction  will  arise  in  this  region 
which  then  travels  towards  its  other  end,  progressively 
involving  its  consecutive  segments.  Thus,  if  two  levers  are 
placed  horizontally  upon  this  muscle  at  an  appreciable  dis- 
tance from  one  another,  the  lever  nearest  the  seat  of  the 
excitation  will  be  moved  first. 

Very  conspicuous  waves  of  contraction  are  discernible 
in  smooth  muscle  tissue.  Such  organs  as  the  stomach, 
intestine,  ureter,  or  bladder  show  characteristic  waves  of 
peristalsis  which  slowly  progress  from  one  zone  to  another. 
A  similar  effect  is  noticeable  in  the  heart,  and  especially  in 
that  of  the  lower  forms,  because  the  contraction  of  this  organ 
is  initiated  at  its  venous  entrance,  whence  it  travels  towards 
its  apical  portion.  This  statement  implies  that  its  vestibular 
portion  completes  its  contraction  before  that  of  the  auricles 
actually  begins.  Likewise,  the  contraction  of  the  auricles  is 
fully  developed  before  that  of  the  ventricles  actually  sets  in. 
It  need  not  be  emphasized  that  this  peristaltic  manner  of 
contraction  is  absolutely  essential  for  the  orderly  flow  of  the 
blood  through  the  different  chambers  of  this  organ. 

The  Bones  as  an  Aid  to  Muscular  Power. — It  has  been 
stated  above  that  the  skeletal  muscles  may  be  arranged  in 
two  groups,  embodying,  on  the  one  hand,  those  which  use 
the  bones  as  levers  to  increase  their  power,  and,  on  the 
other,  those  which  employ  them  merely  as  simple  points  of 
attachment.  It  will  be  remembered  that  a  lever  is  a  rigid 
bar,  one  part  of  which  is  fixed  while  the  other  is  freely  mov- 
able. It  presents  a  part  to  which  the  power  is  applied  to 
overcome  the  obstacle  (P),  a  part  of  support  or  fulcrum  (F), 


THE   MANNER   OF   CONTRACTION   OF   MUSCLE  73 

and  a  point  of  resistance  acting  against  the  weight  or  ob- 
stacle (W).  In  accordance  with  the  relative  positions  of 
these  points,  the  mechanician  recognizes  three  systems  of 
levers:  namely, 

(a)  The  fulcrum  is  situated  between  the  power  and  the 
weight.  As  an  example  of  this  kind  might  be  mentioned 
the  rocking  of  the  head  upon  the  atlas  as  fulcrum.  The 


FIG.  24. — Different  systems  of  levers.     F,  fulcrum ;  P,  power ;  W,  weight. 

muscles  at  the  back  of  the  neck  then  form  the  power,  while 
the  face  represents  the  weight.  This  system  may  also  be 
imitated  by  raising  the  foot  and  tapping  the  floor  with  the 
toes.  In  this  case,  the  fulcrum  lies  at  the  ankle-joint,  and 
the  weight  at  the  toes.  The  gastrocnemius  muscle  forms  the 
power. 

(6)  The  fulcrum  is  placed  at  one  end,  and  the  weight  be- 
tween it  and  the  power.  As  an  example  illustrating  this 
system  might  be  mentioned  the  raising  of  the  body  upon  the 
toes  as  during  the  first  stage  of  stepping  forward.  The  toes 
represent  the  fulcrum,  while  the  power  is  furnished  by  the 
gastrocnemius  muscle.  The  weight  is  applied  at  the  ankle- 
joint. 


74  MUSCLE   AND    NERVE 

(c)  The  fulcrum  lies  at  one  end  and  the  power  between  it 
and  the  weight.  In  illustration  of  this  arrangement  might  be 
mentioned  the  flexion  of  the  forearm  upon  the  arm.  The 
fulcrum  lies  at  the  elbow-joint,  and  the  weight  at  the  hands, 
whereas  the  power  is  furnished  by  the  biceps  muscle.  The 
tendon  of  the  latter  is  inserted  upon  the  radius. 

Usually  the  pull  of  a  muscle  is  exerted  in  a  straight  line 
to  its  axis,  joining  its  points  of  attachment  and  insertion. 
Thus,  the  superior  rectus  of  the  eyeball  moves  the  cornea 
upward,  while  the  inferior  rectus  rolls  it  downward.  Some 
muscles,  however,  do  not  show  this  arrangement,  and  give 
rise  to  a  movement  the  reverse  of  what  might  be  expected. 
For  example,  the  superior  oblique  of  the  eyeball  causes  the 
cornea  to  move  downward,  because  its  tendon  passes  across 
a  pulley-like  structure  before  it  is  inserted  upon  the  eye. 
Another  peculiar  arrangement  is  shown  by  the  digastric 
muscle.  As  the  name  indicates,  this  muscle  consists  of 
two  masses  which  are  joined  by  a  tendinous  part,  the  latter 
being  fastened  to  the  hyoid  bone  by  a  pulley-like  structure. 
Inasmuch  as  this  muscle  extends  from  the  base  of  the  skull 
to  the  anterior  margin  of  the  lower  jaw,  its  contraction  must 
aid  in  separating  the  lips  and  opening  the  mouth. 

Different  Types  of  Movements  Employed  in  Locomotion. — 
The  skeletal  muscles  of  our  body  are  usually  arranged  in  such 
a  way  that  their  actions  oppose  one  another.  This  point 
may  best  be  illustrated  by  referring  to  the  movements  of  the 
forearm  in  consequence  of  the  contraction  of  the  biceps  and 
triceps  muscles.  The  former  is  situated  upon  the  ventral 
surface  of  the  humerus,  while  the  latter  is  placed  upon  its 
dorsal  surface.  When  the  biceps  contracts  the  forearm  is 
moved  upward.  This  constitutes  its  flexion  upon  the  arm. 
If  the  triceps  is  now  activated,  the  forearm  is  again  forced 
downward.  This  represents  its  movement  of  extension. 
Clearly,  therefore,  flexion  and  extension  are  antagonistic 
movements  which  are  made  possible  by  the  activation 
of  only  one  of  these  muscles.  While  one  is  contracted,  the 
action  of  the  other  is  inhibited.  As  far  as  the  human  body  is 
concerned,  flexion  is  usually  accomplished  in  a  forward 
direction.  An  exception  to  this  rule  is  formed  by  the  legs 


THE   MANNER   OF   CONTRACTION   OF   MUSCLE  75 

which  are  flexed  backwards.  Besides  flexion  and  extension, 
we  recognize  the  movements  of  abduction  and  adduction, 
and  rotation  and  circumduction. 

(a)  Flexion  and  Extension. — When  the  point  of  insertion 
of  a  muscle  is  brought  closer  to  its  point  of  attachment,  the 
distant  part  of  the  limb  is  bent  upon  the  central  one.     The 
straightening  out  of  the  limb  constitutes  the  movement  of 
extension.     The  flexion  and  extension  of  the  forearm  upon 
the  arm  comes  to  our  minds  first  when  seeking  to  illustrate 
this  movement. 

(b)  Abduction  and  Adduction. — When  a  part  is  moved 
away  from  the  medium  line  of  the  body,  it  is  abducted,  and 
when  drawn  toward  it,  adducted.     As  an  example  of  this 
type  of  movement  might  be  mentioned  the  abduction  and 
adduction  of  the  thigh. 

(c)  Rotation  and  Circumduction. — A  part  is  rotated  when 
it  is  made  to  turn  upon  its  axis.     For  obvious  reasons,  per- 
fect rotations,  as  exemplified  by  the  wheels  of  a  wagon, 
are  not  possible.     The  acrobat  who  is  able  to  suspend  him- 
self from  a  horizontal  bar  with  his  arms  and  to  rotate  his 
body  in  the  glenoid  cavity  more  than  once,   must  have 
acquired  an  extreme  movability  of  the  head  of  the  humerus 
in  its  capsule,  so  that  he  is  able  to  dislocate  it  at  will.     The 
supination  and  pronation  of  the  hand  belong  in  this  group 
of    movements.     Circumduction    is    accomplished   by   de- 
scribing a  conical  surface  by  rotation  around  an  imaginary 
axis. 


CHAPTER  VII 
ANALYSIS    OF    MUSCULAR    CONTRACTION 

The  Simple  Twitch. — When  a  skeletal  muscle  contracts 
its  point  of  insertion  is  brought  closer  to  its  point  of  attach- 
ment. We  have  seen  that  this  change  is  brought  about  by 
the  shortening  of  the  entire  muscle  and  hence,  also  of  its  indi- 
vidual cells.  In  the  normal  muscle  this  movement  may  be 


FIG.  25. — Mosso's  ergograph.  c,  is  the  carriage  moving  to  and  fro  on 
runners  by  means  of  the  cord  d,  which  passes  from  the  carriage  to  a  holder 
attached  to  the  last  two  phalanges  of  the  middle  finger  (the  adjoining 
fingers  are  held  in  place  by  clamps) ;  p,  the  writing  point  of  the  carria'ge, 
c,  which  makes  the  record  of  its  movements  on  the  kymograph;  w,  the 
weight  to  be  lifted.  (Howell.) 

conveniently  studied  by  means  of  the  apparatus  shown 
in  Fig.  25.  It  is  usually  designated  as  an  ergograph,  and 
consists  of  a  support  for  the  forearm  and  a  weight  which  is 
connected  with  one  of  the  fingers  by  means  of  a  sling  and 
cord.  The  weight  is  equipped  with  a  writing  lever  which  is 

76 


ANALYSIS   OF   MUSCULAR   CONTRACTION  77 

permitted  to  rest  against  the  smoked  paper  of  the  drum  of  a 
kymograph.  When  the  finger  is  alternately  flexed  and 
extended,  the  movements  of  this  part  are  registered  by  the 
weight  upon  the  paper.  The  phase  of  contraction  is  in- 
dicated by  the  upstroke,  and  the  period  of  relaxation  by  the 


FIG.  26. — Normal  fatigue  curve  of  the  flexor  of  the  index  finger.     Weight, 
3  kg.;  contractions  repeated  every  second. 

downstroke  of  the  writing  lever.  Obviously,  these  phases 
appear  upon  the  paper  of  a  stationary  drum  in  the  form  of 
single  vertical  lines.  When,  however,  the  drum  of  the 
kymograph  is  made  to  revolve,  the  upstroke  and  downstroke 
of  the  curve  become  widely  separated  from  one  another,  and 


A  B 

FIG.  27. — Muscle  curves  recorded  with  different  speeds  of  the  drum  of  the 

kymograph. 

the  more  so,  the  greater  the  speed  of  the  drum.     Curves  of 
this  kind  are  represented  in  Fig.  27. 

The  minute  character  of  a  muscular  contraction  may  also 
be  studied  by  fastening  an  excised  muscle  in  a  stand  in  such 
a  way  that  its  tendon  remains  free  to  act  against  a  writing 
lever  adjusted  upon  the  smoked  paper  of  a  kymograph. 


78 


MUSCLE   AND   NERVE 


The  muscle  usually  employed  for  this  purpose  is  the  gastroc- 
nemius  of  a  recently  killed  frog.  Its  upper  pole  is  securely 
fixed  in  a  clamp,  while  its  tendon  is  connected  with  the  writ- 
ing lever  distally  to  its  center  of  rotation.  A  slight  weight  is 
then  attached  to  the  lever  directlv  underneath  the  tendon. 


FIG.  28. — A  method  used  to  register  muscular  contraction.  St,  stand 
for  holding  of  clamp  C  and  writing  lever.  WL,  the  muscle  M  is  attached 
to  the  lever  by  means  of  a  small  hook  and  string.  The  lever  is  counter- 
poised by  weight  W.  The  stimulation  is  effected  through  the  electrodes, 
S.  The  speed  of  the  kymograph  K  may  be  varied  by  fan  F. 

When  contracting,  the  muscle  raises  the  lever  to  a  certain 
height,  but  again  lowers  it  during  its  subsequent  phase  of 
relaxation.  The  greater  the  speed  of  the  drum,  the  more 
widely  will  these  two  limbs  of  the  curve  be  separated  from 
one  another. 

We  have  previously  noted  that  a  muscle  in  situ  is  stimu- 
lated under  ordinary  circumstances  in  an  indirect  manner  by 
impulses  conveyed  to  it  through  its  motor  nerve.  But  it  is 
also  possible  to  activate  it  in  a  direct  way  by  applying  artifi- 
cial stimuli  to  the  skin  overlying  it.  An  excised  muscle, 


ANALYSIS   OF   MUSCULAR    CONTRACTION 


79 


on  the  other  hand,  can  only  be  made  to  contract  by  artificial 
stimuli,  and  these  stimuli  may  be  passed  into  it  by  bringing 
the  wires  from  the  battery  in  direct  contact'  with  its  sub- 
stance or  that  of  its  nerve.  Electrical  stimuli  are  employed 
because  they  are  more  easily  produced  and  applied  than  the 
others.  The  time  which  the  muscle  requires  for  its  contrac- 


Tib.  ant.  long. 


Sartorius 

Add.  magn. 
Gracilis 


Tendo  Achillis 


FIG.  29. — The  muscles  of  the  hind  leg  of  frog.     (Baker.) 


tion,  may  be  ascertained  by  permitting  a  tuning  fork  to 
register  its  vibrations  below  the  line  of  the  muscle  lever. 
Lastly,  it  is  advisable  to  record  the  moment  when  the  elec- 
trical shock  is  sent  into  the  muscle  or  its  nerve  by  means  of 
an  electro-magnetic  signal  inserted  in  the  battery  circuit. 
When  a  muscle  is  made  to  register  its  contraction  under 


80  MUSCLE    AND    NERVE 

these  circumstances,  it  yields  a  curve  such  as  is  represented 
in  Fig.  30.  This  curve  consists  of  two  principal  phases, 
representing  the  periods  of  shortening  (C)  and  relaxation  (R) 
of  the  muscle.  If  a  comparison  is  now  made  between  the 
beginning  of  the  phase  of  contraction  and  the  moment  of 
stimulation  (M),  it  will  be  noted  that  the  muscle  does  not 
react  precisely  when  stimulated  but  a  little  later.  The  time 
elapsing  between  the  moment  of  stimulation  and  the  onset  of 


FIG.  30. — A  muscle  twitch.  M,  make  shock  recorded  by  magnetic 
signal  connected  with  primary  circuit.  Time  in  }{ oo  sec.;  L,  latent 
period;  C,  period  of  contraction;  R,  period  of  relaxation. 

the  reaction,  constitutes  the  latent  period  (L).  During  this 
phase  diverse  processes  are  promulgated  in  its  substance, 
which  finally  give  rise  to  the  shortening  of  the  muscle  as  a 
whole. 

The  time  during  which  a  muscle  is-  able  to  complete  its 
contraction,  differs  with  its  condition  at  the  time  of  experi- 
mentation. If  we  confine  ourselves  at  this  time  to  the  gastro- 
cnemius  muscle  of  the  frog,  it  will  be  found  that  its  latent 
period  usually  consumes  0.01  sec.,  its  period  of  contraction 
0.04  sec.,  and  its  period  of  relaxation  0.05  sec.  A  rapid  con- 
traction of  this  kind  following  a  single  stimulus,  is  known  as 
a  twitch.  It  should  be  remembered,  however,  that  this 
time  of  0.1  sec.  is  by  no  means  the  shortest  recorded  in  any 
muscle,  because  the  wing-muscles  of  the  insects  contract  two 
and  three  hundred  times  in  a  second. 

Summation  of  Contraction. — If  a  second  stimulus  is 
applied  to  the  muscle  shortly  after  the  beginning  of  its 


ANALYSIS  OF  MUSCULAR  CONTRACTION        81 

period  of  relaxation,  a  second  contraction  will  be  obtained 
which  is  higher  than  the  first.  In  the  same  manner  a  third 
contraction  may  be  mounted  upon  the  second,  and  so  on, 


FIG.  31. — Summation  of  contractions.  M  and  B,  make  and  break 
shocks  indicated  by  an  electro-magnetic  signal.  Time  in  ;HlOO  sec.  As 
the  break  contraction  occurs  during  the  period  of  relaxation  of  the  make 
contraction,  it  is  added  to  the  first. 


FIG.  32. — Fusion  and  tetanus.  S,  summation;  F,  fusion;?1,  tetanus. 
Time  in  seconds.  The  individual  make  and  break  shocks  are  repeated  so 
quickly  that  a  continuous  contraction  is  obtained. 

until  the  individual  contractions  become  partially  fused  into 
an  incomplete  tetanus. 

Tetanic  Contraction. — If  the  individual  electrical  shocks 
are  repeated  at  a  still  more  rapid  rate,  the  muscle  cannot 


82  MUSCLE   AND   NERVE 

relax  at  all  and  remains  in  a  state  of  maximal  contraction. 
The  incomplete  tetanus  is  then  changed  into  a  complete 
tetanus.  Ordinarily  about  20  to  50  stimuli  in  a  second  are 
required  to  tetanize  the  gastrocnemius  of  the  frog.  It  is 
true,  however,  that  the  contractile  material  of  this  muscle  is 
rapidly  used  up  during  this  prolonged  form  of  contraction, 


FIG.  33. — Tetanic  contraction.  Recorded  by  means  of  Neef's  auto- 
matic interrupter.  Timj  in  seconds.  The  decline  of  the  curve  is  an 
indication  of  fatigue. 

and  that  this  partial  depletion  of  its  energy  store  finally 
induces  a  certain  degree  of  fatigue  which  is  indicated  in  the 
curve  by  a  slow  decrease  in  its  height.  When  completely 
fatigued,  the  muscle  returns  into  its  position  of  relaxation  in 
spite  of  the  continuance  of  the  excitation. 

Repeated  determinations  of  the  contraction-time  of  human 
muscles  have  shown  that  they  do  not  respond  so  swiftly  as 
the  muscles  of  the  cold-blooded  animals.  Even  such  a  brief 
muscular  response  as  is  required  to  close  the  eyelids  is  tetanic 
in  its  nature.1  The  same  statement  may  be  made  regarding 
the  trained  movements  of  the  fingers  of  a  pianist  or  typist. 
A  speed  of  ten  in  a  second  is  rarely  attained.  Accordingly, 
it  is  believed  that  the  contractions  of  our  skeletal  muscles 
are  the  result  of  a  series  of  nervous  stimuli  and. not  of  a 
solitary  one.  Thus,  it  will  be  seen  that  if  a  certain  muscle  is 

:In  medical  literature  the  name  tetanus  refers  to  lock-jaw.  In 
this  disease  the  muscles,  and  especially  those  attached  to  the  lower 
jaw,  become  firmly  contracted. 


ANALYSIS   OF  MUSCULAR   CONTRACTION  83 

to  be  contracted,  the  corresponding  motor  nerve  cells  send 
out  rhythmic  discharges  which  do  not  cease  flowing  until 
the  contraction  is  to  be  discontinued.  These  cells,  therefore, 
may  be  compared  to  batteries  discharging  through  a  vibrator. 

The  closest  approach  to  a  simple  twitch  is  presented  by 
cardiac  muscle.  Smooth  muscle  tissue  responds  very  slug- 
gishly. All  of  its  phases  are  very  much  prolonged,  so  that 
the  latent  period  may  occupy  several  seconds  instead  of  a 
few  hundred ths  of  a  second.  The  total  length  of  its  contrac- 
tion depends,  of  course,  upon  the  intensity  of  the  stimulus, 
but  assuming  that  such  a  preparation  as  the  cat's  bladder 
has  been  subjected  to  a  tetanizing  current  of  moderate 
strength  and  duration,  it  may  take  3  to  6  minutes  before  its 
muscle  cells  again  assume  their  original  length. 

The  Factors  Modifying  the  Height  of  Contraction. — It  is  a 
matter  of  common  experience  that  the  amplitude  of  the 


»0        II        II       13 


FIG.  34. — Successive  make  and  break  contractions.  The  strength  of 
the  current  is  gradually  diminished  by  more  widely  separating  the  second- 
ary from  the  primary  coil.  The  figures  indicate  this  separation  in  centi- 
meters of  distance.  M,  threshold  of  make;  B,  threshold  of  break. 

contractions  of  heart  muscle  remains  practically  the  same, 
whereas  that  of  the  contractions  of  striated  and  smooth 
muscle  may  be  varied  considerably.  The  principal  factor 
responsible  for  this  change  is  the  strength  of  the  stimulus. 
If  an  excised  striated  muscle  is  stimulated  successively  with 
single  shocks  of  decreasing  intensity,  a  curve  is  obtained 
such  as  is  represented  in  Fig.  34.  This  proves  that  the 
amplitude  of  reaction  decreases  steadily  with  the  stimulus, 
although  it  is  noticeable  that  supramaximal  stimuli  yield 
smaller  responses  than  those  of  maximal  value.  It  seems 
that  a  mild  stimulus  affects  only  a  limited  number  of  cells, 


84  MUSCLE    AND    NERVE 

whereas  a  strong  one  activates  a  much  larger  number. 
This  difference  is  not  exhibited  by  cardiac  muscle,  because 
its  cells  are  joined  by  short  processes  which  permit  the  wave 
of  excitation  to  spread  rapidly  from  fiber  to  fiber.  For  this 
reason,  heart  muscle  always  reacts  maximally,  a  fact  which  is 
commonly  expressed  by  saying  that  this  tissue  follows  the 
all-or-none  law  of  contraction. 

It  may  be  stated  in  a  general  way  that  a  lasting  stimulus  is 
a  more  efficient  exciting  agent  than  a  brief  one,  but  this  re- 
lationship holds  true  only  as  long  as  its  duration  is  not  unduly 
prolonged.  In  the  latter  event,  fatigue  rapidly  diminishes 
the  amplitude  of  the  contractions.  As  might  be  expected, 
increasing  weights  gradually  reduce  the  height  of  muscular 
contractions.  It  is  evident,  however,  that  a  slight  load 
exerts  a  favorable  influence,  because  it  augments  the  elastic 
tension  of  the  contractile  elements  (Fig.  35). 


O     \Q    ZO    30   «K)    SO    t>0    ^0    80    <JO  JQO 

FIG.  35. — Influence  of  load.     This  muscle  has  been  successively  loaded 
with  10  gram  weights. 

The  character  of  the  muscle  substance  is  another  important 
factor,  because  upon  it  are  based  the  power  and  rapidity  of 
the  reaction  of  the  muscle.  For  example,  since  striated 
muscle  contains  a  larger  amount  of  water  and  smaller  amount 
of  undifferentiated  sarcoplasm  than  smooth  muscle,  it  is  able 
to  respond  more  swiftly  than  the  latter.  In  this  connection 
attention  is  also  called  to  the  fact  that  striated  muscle  is 
frequently  made  up  of  fibers  of  different  quality,  its  "pale" 
constituents  reacting  more  rapidly  than  its  "dark"  ones. 

Warmth   increases   the   amplitude   of   the    contractions, 


ANALYSIS   OF   MUSCULAR   CONTRACTION  85 

whereas  cold  decreases  it.  This  may  easily  be  proved  by 
placing  a  gastrocnemius  muscle  in  a  compartment,  the  tem- 
perature of  which  may  be  changed  at  will.  Fig.  36  represents 


FIG.  36. — Effect  of  changes  in  the  temperature  on  muscular  contraction. 
The   temperature   was   raised    5°   each   time. 

a  series  of  tracings  taken  at  differences  of  5°  C.  It  will  be 
seen  that  those  recorded  at  low  temperatures  are  very  slug- 
gish, while  those  registered  at  about  30°  C.  are  very  rapid. 
It  is  also  to  be  remembered  that  the  muscles  of  the  frog  begin 
to  lose  their  irritability  at  about  37°  C.;  and  finally  pass  into 
the  state  of  heat-rigor  or  rigor  caloris  (42°  C.).  When  in 
this  condition  the  muscle  possesses  on  opaque  appearance 
and  is  maximally  shortened.  It  cannot  be  made  to  contract 
again.  The  optimum  temperature  for  the  muscles  of  warm- 
blooded animals  is  37°  C. 

Certain  chemicals  affect  the  irritability  and  contractility 
of  striated  muscle  in  a  most  peculiar  manner.  Veratrin 
greatly  prolongs  its  phase  of  relaxation.  Potassium  salts 
act  as  depressants,  whereas  sodium  and  calcium  salts  possess 
an  excitatory  action.  A  solution  of  0.7  per  cent,  sodium 
chlorid  is  employed  to  keep  the  muscle  in  the  best  possible 
condition,  because  it  is  isotonic  to  its  substance.  Common 
stimulants  of  cardiac  muscle  are  caffein  and  strychnin. 
Adrenalin  diminishes  muscular  fatigue,  owing  to  its  stimulat- 
ing action  upon  the  circulation. 


CHAPTER  VIII 
THE  CHEMISTRY  OF  MUSCLE 

Fatigue  of  Muscle. — If  an  excised  muscle  is  stimulated 
a  great  many  times,  the  height  of  the  successive  contrac- 
tions gradually  decreases,  until  the  writing  lever  remains 
finally  in  the  horizontal  position.  The  length  of  the  individ- 
ual curves,  however,  is  materially  increased.  This  gradual 
decline  in  the  power  of  the  muscle  to  respond  to  stimuli  is 
known  as  fatigue.  It  is  to  be  remembered,  however,  that  a 
muscle  on  repeated  contraction  shows  first  a  slight  augmen- 
tation before  its  reactions  are  actually  diminished.  This 
brief  initial  gain  in  height  corresponds  to  the  process  of 
''warming  up"  of  the  normal  skeletal  muscles  of  the  higher 
animals.  It  is  also  of  interest  to  note  that  a  fresh  gastroc- 
nemius  muscle  is  able  to  contract  more  than  one  thousand 
times  before  it  becomes  functionally  useless.  Moreover, 
this  condition  of  fatigue  is  permanent,  i.e.,  the  muscle  cannot 
be  made  to  contract  again  even  after  a  long  period  of  rest. 
This  result  suggests  that  its  store  of  contractile  material 
has  been  completely  exhausted  and,  naturally,  an  excised 
muscle  possesses  no  means  of  rebuilding  the  substances  which 
it  has  lost  during  its  preceding  reactions. 

Since  a  normal  muscle  is  in  a  very  favorable  position  to 
replenish  its  contractile  material,  it  cannot  be  so  thoroughly 
fatigued  as  an  excised  one.  But,  there  is  another  factor 
which  plays  an  important  part  in  the  production  of  this 
phenomenon,  and  that  is  the  accumulation  of  the  waste 
products  formed  in  the  course  of  muscular  metabolism.  An 
engine  may  stop  on  account  of  a  shortage  of  fuel,  but  also 
because  its  exhaust  pipes  have  become  clogged  with  the 
products  of  combustion.  A  normal  muscle*  is  constantly 
flushed  out  with  blood  containing  nutritive  substances  and 
oxygen.  In  the  presence  of  this  gas  the  waste  products 


THE   CHEMISTRY   OF   MUSCLE  87 

formed  in  the  course  of  muscular  contraction,  are  quickly 
oxidized  and  excreted.  Since  an  excised  muscle  is  not  in 
possession  of  an  unlimited  supply  of  oxygen,  it  cannot  rid 
itself  of  these  injurious  substances.  Much  of  this  elimina- 
tion is  accomplished  during  bodily  rest,  a  state  diametrically 
opposed  to  the  state  of  work,  and,  naturally,  the  most 
complete  repair  is  had  during  sleep  when  the  muscles  are 
fully  relaxed  and  no  demand  is  made  upon  their  store  of 
energy. 

The  Chemistry  of  Muscle. — The  importance  of  the  muscles 
in  animal  economy  is  clearly  betrayed  by  the  fact  that  about 
42  per  cent,  of  our  weight  is  due  to  this  tissue.  In  further 
illustration  of  this  point,  it  might  be  mentioned  that  about 
50  per  cent,  of  the  total  metabolism  of  a  resting  person  is 
apportioned  to  this  tissue,  and  that  a  person  in  moderate 
activity  may  attain  a  muscular  metabolism  of  as  much  as 
75  per  cent,  of  the  total.  These  values  immediately  bring 
up  the  question  regarding  the  character  of  the  substances  in 
normal  muscle  which  may  be  held  responsible  for  this  sur- 
prisingly large  production  of  energy.  While  the  succeeding 
table  illustrating  the  composition  of  muscle  tissue  may  be  of 
service  in  answering  this  query,  this  subject-matter  cannot  be 
fully  understood  until  the  contents  of  the  subsequent  para- 
graphs have  been  noted. 

Water 74 . 0  per  cent. 

Solids 26.0  per  cent. 

Proteins 18.0  per  cent. 

Collagen 2.0  per  cent. 

Fat 2.0  per  cent. 

Glycogen less  than  1 . 0  per  cent. 

Creatin 0.3  per  cent. 

.     Other  organic  substances 

Inorganic  substances 3.0  per  cent. 

The  Chemistry  of  Contracting  Muscle. — The  metabolic 
changes  in  muscle  on  contraction  are  characterized  by  a 
constancy  of  the  catabolism  of  the  proteins  and  an  increase 
in  the  catabolism  of  the  carbohydrates.  Accordingly, 
muscular  work  is  not  followed  by  a  greater  nitrogen  output, 
but  by  a  disappearance  of  glycogen,  a  substance  closely 


88  MUSCLE    AND   NERVE 

allied  to  ordinary  starch.  This  change  is  associated  with  a 
production  of  lactic  acid  and  elimination  of  much  larger 
amounts  of  carbon  dioxid.  It  must  be  evident,  therefore, 
that  the  energy  liberated  by  muscle  is  chiefly  derived  from 
the  carbohydrates  of  the  food. 

The  production  of  carbon  dioxid  is  indicated  by  the  fact 
that  the  expired  air  contains  larger  amounts  of  this  gas.  To 
be  sure,  the  oxygen  intake  is  also  augmented,  but  the  absorp- 
tion of  this  gas  remains  below  the  output  of  carbon  dioxid. 
It  is  a  matter  of  common  experience  that  muscular  exercise 
is  always  accompanied  by  a  greater  respiratory  activity. 


FIG.  37. — Fatigue  of  muscle.  A  gastrocnemius  muscle  of  the  frog 
stimulated  successively  150  times.  The  1st,  50th,  100th,  and  150th 
contractions  are  recorded. 

Resting  muscle  is  neutral  or  faintly  alkaline  in  its  reaction, 
whereas  active  muscle  is  distinctly  acid.  This  acidity  finds 
its  origin  in  an  accumulation  of  sarcolactic  acid.  Conse- 
quently, much  larger  amounts  of  this  acid  must  be  present 
in  an  excised  muscle  than  in  one  able  to  obtain  fresh  oxygen, 
because  in  the  presence  of  this  gas  this  waste  product  is 
quickly  reduced.  This  diminution  in  the  store  of  glycogen 
may  be  proved  in  a  quantitative  way.  Thus,  it  has  been 
found  that  frog's  muscle  loses  from  24  to  50  per  cent,  of  its 
glycogen  when  tetanized,  while  a  resting  muscle  contains 
considerable  amounts  of  this  substance. 

The  Fatigue  Substances. — We  have  previously  noted 
that  the  excessive  stimulation  of  a  muscle  causes  it  to  lose  its 
irritability  and  contractility.  Even  a  very  strong  stimulus 
is  then  no  longer  able  to  activate  it.  This  functional  exhaus- 
tion has  been  referred  to  two  causes :  namely,  an  inadequate 
acquisition  of  energy  yielding  material  and  an  accumulation 


THE    CHEMISTRY    OF    MUSCLE  89 

of  certain  waste  products.  The  latter  are  frequently  desig- 
nated as  fatigue  substances.  The  fact  that  such  bodies  are 
actually  liberated  and  enter  the  circulation,  has  been  proved 
by  permitting  the  blood  of  a  fatigued  animal  to  flow  into  the 
circulatory  channels  of  a  perfectly  normal  one.  The  latter 
then  exhibited  the  phenomena  of  fatigue  as  if  it  itself  had 
been  subjected  to  excessive  muscular  exercise. 

The  principal  fatigue  substance  is  an  acid  which  is  similar 
in  its  constitution  to  the  acid  formed  in  the  process  of  souring 
milk.  It  is  usually  called  sarcolactic  acid  to  distinguish  it 
from  the  lactic  acid  of  milk.  Other  fatigue  substances  are 
carbon  dioxid  and  acid  potassium  phosphate.  It  has  also 
been  noted  that  certain  toxic  bodies  are  produced,  but  this 
point  has  not  been  definitely  settled  as  yet.  It  is  obvious 
that  these  so-called  fatigue  substances  are  liberated  in  the 
course  of  muscular  metabolism,  and  are  able  to  evoke  this 
characteristic  action  because  they  are  only  partially  oxidized. 
The  symptoms  resulting  in  consequence  of  the  accumulation 
of  these  metabolites  are  local  as  well  as  general  in  their 
character,  i.e.,  besides  the  localized  feeling  of  discomfort  and 
pain,  one  also  experiences  a  general  lassitude  of  body  and 
mind.  But  while  muscular  fatigue  usually  induces  mental 
fatigue,  it  may  also  happen  that  these  events  are  reversed. 
Thus,  we  well  know  that  mental  fatigue  is  usually  associated 
with  a  lassitude  of  the  entire  body.  The  manner  in  which 
this  reaction  is  brought  about  is  not  quite  clear,  although 
it  is  entirely  probable  that  the  nervous  tissue  generates  a 
set  of  catabolic  products  very  similar  to  those  of  muscle 
tissue.  It  may  be  concluded,  therefore,  that  this  relation- 
ship between  the  muscles  and  the  central  nervous  system  is 
dependent  not  only  upon  nervous  reflexes  but  also  upon 
definite  chemical  substances. 

The  excessive  exercise  of  the  muscles  when  not  accustomed 
to  it,  frequently  gives  rise  to  a  complex  of  symptoms,  the 
most  annoying  of  which  is  a  painful  stiffness  of  the  muscula- 
ture. In  addition,  one  may  experience  a  general  discomfort, 
inability  to  work,  lassitude,  and  a  slight  febrile  reaction. 
Pertaining  to  the  cause  of  the  local  symptoms,  it  is  commonly 
stated  that  they  find  their  origin  in  the  excessive  displace- 


90  MUSCLE   AND   NERVE 

ment  of  the  muscle  fibers  and  the  " drying  up'7  of  the  inter- 
cellular material.  Very  similar  results  often  follow  the 
violent  manipulation  of  the  muscles  with  the  fingers,  as 
during  massage.  The  general  symptoms  are  referable  to 
the  toxic  action  produced  by  the  fatigue  substances. 

The  Chemistry  of  Rigor  Mortis. — It  is  a  well  known  fact 
that  the  muscles  of  the  mammals  become  perfectly  rigid 
very  shortly  after  the  blood  has  ceased  to  circulate.  This 
condition  which  is  known  as  death-rigor,  or  rigor  mortis, 
lasts  for  a  certain  period  of  time  and  then  disappears.  Dis- 
solution setting  in  the  muscles  resume  their  soft  consistency, 
but  cannot  be  made  to  contract  again.  Inasmuch  as  the 
skeletal  muscles  are  frequently  arranged  in  an  antagonistic 
manner,  their  simultaneous  rigidity  must  give  rise  to  an 
inflexibility  of  the  part  normally  moved  by  them.  For  this 
reason,  the  arms  and  legs  cannot  be  bent  at  this  time  unless 
the  tendons  of  their  .muscles  are  torn. 

The  time  required  for  the  development  of  rigor  mortis  var- 
ies considerably.  Most  generally,  it  appears  in  the  course  of  a 
few  hours,  but  sometimes  only  after  about  twenty-four  hours. 
In  cases  of  extensive  laceration  of  the  central  nervous  system, 
it  has  been  known  to  set  in  almost  instantaneously.  The 
duration  of  death  rigor  also  varies  materially.  Sometimes 
it  terminates  within  a  few  hours,  and  sometimes  only  after 
several  days.  Several  factors  are  at  work  to  cause  this 
divergency;  for  example,  the  temperature,  the  condition  of 
the  body  at  the  time  of  death,  the  age  of  the  individual,  and 
the  type  of  the  lesion  terminating  life. 

It  is  believed  that  rigor  mortis  is  due  to  the  coagulation 
of  the  protein  material  of  muscle.  The  soluble  proteins  are 
temporarily  converted  into  their  insoluble  form  under  the 
influence  of  lactic  acid.  When  the  acidity  has  again  been 
destroyed  by  the  slow  oxidation  of  the  lactic  acid,  the  reverse 
chemical  change  permits  the  muscles  to  acquire  their 
former  soft  texture. 

The  Production  of  Energy  in  Muscle. — When  a  muscle 
contracts,  its  stored  energy  is  liberated  to  yield  certain 
mechanical  changes.  But  before  this  evolution  of  mechani- 
cal energy  can  be  repeated,  the  muscle  must  oxidize  a  certain 


THE    CHEMISTRY   OF   MUSCLE  91 

amount  of  material  and  store  it  in  some  form  ready  for  the 
next  contraction.  During  this  process  of  acquiring  stored 
energy,  it  liberates  heat  and  a  small  amount  of  electricity. 
It  appears,  therefore,  that  these  forms  of  energy  are  not 
evolved  simultaneously,  but  successively,  and  that  the  oxida- 
tions really  succeed  the  actual  contractions,  their  purpose 
being  to  reform  the  material  lost  during  the  preceding  period 
of  activity.  In  building  a  pier  the  individual  posts  are  driven 
into  the  ground  by  letting  a  weight  drop  upon  them.  The 
potential  or  resting  energy  contained  in  the  weight  suspended 
high  up  between  its  guides,  is  converted  into  kinetic  energy 
by  its  fall.  This  phase  corresponds  to  the  period  of  contrac- 
tion of  the  muscle  and  its  resultant  liberation  of  mechanical 
energy.  But,  before  the  weight  can  again  produce  an  im- 
pact, it  must  first  be  raised  to  its  former  height  by  steam 
power.  Quite  similarly,  a  muscle  must  first  be  placed  in  a 
proper  condition  for  its  succeeding  mechanical  action  by 
oxidations.  During  this  reaction  heat  and  electricity  are 
evolved. 

As  a  rule,  about  33  per  cent,  of  the  total  energy  of  muscle  is 
liberated  as  mechanical  energy,  but  this  percentage  may  be 
raised  somewhat  by  placing  the  muscle  under  the  best 
possible  conditions  for  work.  In  order  that  muscle  tissue 
may  actually  furnish  work,  it  must  be  weighted  and  raise 
this  weight  to  a  certain  height.  Clearly,  if  it  contracts 
without  a  load,  it  cannot  change  external  conditions  and 
hence,  cannot  yield  any  work.  Likewise,  it  cannot  furnish 
work  if  loaded  with  a  weight  so  heavy  that  it  cannot  lift  it. 
In  this  instance,  most  of  the  energy  liberated  by  it  is  con- 
verted into  heat. 

Under  ordinary  circumstances,  we  ascertain  the  work  per- 
formed by  a  muscle  by  multiplying  the  weight  by  the  height 
to  which  it  has  been  lifted.  The  product  W  is  expressed  in 
milligram-meters.  Thus,  if  25  grams  have  been  raised  to  a 
height  of  10  millimeters,  as  determined  by  the  height  of  the 
curve  recorded  upon  the  paper  of  the  kymograph,  the  muscle 
has  accomplished  250  gram-millimeters  of  work.  In  this 
calculation,  however,  an  allowance  should  be  made  for  the 
magnification  of  the  writing  lever,  as  follows:  L:H::l:h. 


92  MUSCLE   AND   NERVE 

In  this  formula  L  indicates  the  total  length  of  the  lever,  I 
the  length  of  its  short  lever  from  the  axis  to  the  point  of 
attachment  of  the  muscle,  H  the  height  of  the  contraction, 
and  h  the  actual  height  to  which  the  load  has  been  lifted. 

The  heat  produced  by  muscle  may  be  ascertained  by  means 
of  different  appliances  which,  however,  cannot  be  described 
in  detail  at  this  time.  It  is  of  importance  to  remember 
that  the  muscles  are  the  principal  source  of  the  body-heat, 
because  they  form  40  per  cent,  of  the  weight  of  our  body 
and  possess  a  very  intense  metabolism.  It  is  a  matter  of 
common  experience  that  our  body-temperature  may  be 
raised  several  degrees  by  simply  indulging  in  moderate 
muscular  exercise  for  a  relatively  brief  period  of  time,  but 
the  heat  so  generated  is  again  dissipated  within  a  few  minutes. 
Consequently,  a  lasting  effect  cannot  be  produced  in  this 
way.  This  fact  merely  reveals  the  paramount  importance 
of  the  muscles  as  heat-producing  organs.  Entirely  in 
agreement  with  it,  a  person  who  is  about  to  perform  muscular 
work,  selects  a  cool  room  and  wears  relatively  thin  clothing, 
so  as  to  be  able  to  transfer  his  heat  more  rapidly  to  the  air. 
At  the  end  of  the  exercise,  however,  he  should  protect  himself 
well  against  an  excessive  loss  of  heat  and  its  consequences, 
for  as  his  skin  is  moistened  with  perspiration,  he  is  in  a  rela- 
tively unfavorable  position  to  counteract  heat  radiation. 
Woolens  are  well  adapted  to  conserve  the  body-heat,  because 
they  are  hygroscopic  and  do  not  allow  the  sweat  to  evaporate 
too  rapidly. 

An  active  muscle  also  presents  certain  changes  in  its 
electrical  potential.  Since  the  contraction  traverses  its 
substance  in  the  form  of  a  wave,  one  pole  of  it  must  be  active 
while  the  other  is  resting.  By  the  use  of  very  sensitive 
electrical  measuring  devices,  it  has  been  ascertained  that 
the  resting  portion  of  a  contracting  muscle  is  electro-positive 
to  its  active  portion.  This  subject-matter  will  again  be 
considered  later  on  in  connection  with  the  electrical  variations 
occurring  in  the  beating  heart.  This  organ  exhibits  very 
characteristic  differences  in  its  electrical  potential,  which 
may  be  accurately  registered  by  means  of  an  instrument, 
known  as  the  string-galvanometer. 


CHAPTER  IX 
THE  NERVE  IMPULSE  AND  REFLEX  ACTION 

The  Structure  of  Nervous  Tissue. — The  tissue  composing 
the  nervous  system,  consists  of  a  supporting  framework  and 
numerous  generating  and  conductile  elements  or  neurones. 
The  reticular  tissue  appears  in  the  form  of  investments  and 
membraneous  partitions  of  connective  tissue,  enclosing  a 
network  of  spider-like  cells  which  are  known  as  neuroglia  or 
glia  cells.  In  the  spaces  formed  by  the  latter  are  situated 
the  neurones,  the  building  stones  of  the  nervous  system. 

Every  neurone  or  nerve-cell  is  composed  of  a  cell-body  and 
several  well  differentiated  processes.  The  cell-body  may  be 
round,  oval,  stellate,  pyramidal,  or  pear-shaped,  and  pos- 
sesses a  diameter  varying  between  10  and  1 50/i.  Its  cytoplasm 
is  faint  granular  in  appearance,  and  contains  a  very  con- 
spicuous nucleus  and  nucleolus,  as  well  as  dark  irregular 
bodies  which  are  known  as  NissFs  granules.  The  processes 
with  which  the  cell-body  is  equipped,  are  designated  as 
dendrites  and  axones.  This  differentiation  is  made  chiefly 
upon  histological  grounds,  because  the  former  appear  as 
short,  irregular,  many-branched  offshoots  which  remain  in 
the  immediate  vicinity  of  the  cell-body,  whereas  the  latter 
are  long,  and  pursue  a  rather  straight  course.  They  give 
off  very  few  branches  which  are  known  as  collaterals. 
Moreover,  while  a  nerve-cell  may  be  in  possession  of  many 
dendrites,  it  usually  exhibits  only  one  axone.  The  former 
serve  as  points  of  entrance  for  the  nerve  impulses  coming  from 
different  neighboring  cells,  while  the  latter  constitutes  its 
distributing  channel. 

Naturally,  these  processes  extend  for  different  distances 
through  the  nervous  system,  but  if  we  select  for  purposes 
of  illustration  the  path  which  connects  a  muscle  of  the 
foot  with  its  motor  cells  in  the  cerebrum,  we  will  find  that 

93 


94 


MUSCLE    AND    NERVE 


these  extreme  points  of  the  body  are  brought  into  functional 
relation  by  only  two  neurones.  The  cell-body  of  the  first 
lies  in  the  motor  area  of  the  cerebrum,  while  its  axone  passes 
outward  through  the  spinal  cord  as  far  as  the  lumbar  region. 
At  this  level,  it  breaks  up  into  its  terminals  which  in  turn  are 
connected  by  contact  with  the  dendrites  of  the  cell-body  of 


-  A 


FIG.  38. — Purkinje  cell  from  human  cerebellum, 
staining.      (Stohr.) 


Golgi's  method  of 


the  second  neurone.  The  axone  of  the  latter  then  follows 
the  highway  of  the  sciatic  nerve  to  the  foot.  Accordingly, 
each  neurone  must  measure  close  to  one  meter  in  length. 

In  the  case  now  under  consideration,  the  cell-body  is  placed 
at  one  end  of  the  neurone.  This  arrangement  is  always 
present  in  neurones  which  convey  the  nerve  impulse  in  a 
direction  from  center  to  periphery.  They  are  known  as 


THE   NERVE    IMPULSE   AND    REFLEX   ACTION 


95 


motor  or  efferent  neurones.  Those  nerve-cells,  on  the  other 
hand,  which  conduct  impulses  from  the  periphery  to  the 
center,  display  their  cell-bodies  at  some  distance  from  their 
terminals.  They  are  known  as  'sensory  or  afferent  neurones 
(Fig.  39). 


FIG.  39. — M,  motor  neurone;  S,  sensory  neurone;  M,  motor  end- 
organ;  S,  sensory  end-organ;  A,  axis  cylinder;  MS,  myelin  sheath;  N, 
neurilemrna;  C,  collateral;  CB,  cell-body;  D,  dendrites;  Nu,  nucleus  and 
nucleolus;  R,  sensory  terminals. 

Most  generally,  these  sensory  and  motor  neurones  are 
joined  in  such  a  way  that  the  terminal  fibers  of  the  former 
come  to  lie  in  close  contact  with  the  dendrites  of  the  latter. 


96 


MUSCLE   AND    NERVE 


Such  a  point  of  union  between  neighboring  neurones  is 
termed  a  synapse.  This  adjustment  permits  the  sensory  or 
afferent  impulse  to  activate  the  adjoining  motor  or  efferent 
neurone.  It  is  to  be  remembered,  however,  that  the  sensory 
impulse  does  not  actually  pass  through  the  synapse  and 


if" 


ATI 


$0 

FIG.  40. — Reflex  circuit.  SO,  sensory  end-organ,  receptor;  MO,  motor 
end-organ,  effector;  AN,  afferent  neurone;  EN,  efferent  neurone;  C,  center; 
S,  synapse. 

continue  through  the  motor  neurone  as  an  efferent  impulse, 
but  ends  in  the  sensory  terminals.  Hence,  the  dendrites  of 
the  next  motor  neurone1  are  activated  solely  by  contact 

xThe  term  motor  refers  more  particularly  to  those  efferent 
neurones  which  induce  movement,  i.e.,  to  musculo-motor  fibers  and 
nerves.  Certain  efferent  nerves,  however,  evoke  secretion,  in  which 
case  they  are  designated  as  secreto-motor  nerves.  They  may  also 
control  the  discharge  of  electricity  from  special  organs,  as  in  the  elec- 
trical fish,  or  give  rise  to  an  erection  of  the  hairs,  or  a  change  in  the 
caliber  of  the  blood  vessels.  The  terms  of  electro-motor,  pilo-motor, 
and  vaso-motor  are  then  employed  to  characterize  their  function. 


THE    NERVE    IMPULSE    AND  /REFLEX    ACTION  97 

CiXA-vvC^-OML  t.      ^^^  fa   C  U  c"?- 

and  in  a  purely  physical  manner.  The  sensory  impulse, 
therefore,  is  distinct  from  the  motor  impulse. 

The  structural  arrangement  in  the  synapse  may  be  illus- 
trated by  placing  the  fingers  of  the  left  hand  in  the  form  of  a 
basket  around  the  index  finger  of  the  right  hand.  The  latter 
then  occupies  the  position  of  the  terminal  of  the  sensory 
neurone,  and  the  former  those  of  the  receiving  dendrites 
of  the  motor  neurone.  By  separating  these  filaments  of  the 
synapse  more  widely  from  one  another,  a  break  may  eventu- 
ally be  established  between  them,  so  that  they  can  no  longer 
give  rise  to  the  aforesaid  transfer  of  impulses.  For  this 
reason,  it  is  frequently  held  that  the  loss  of  sensation  follow- 
ing the  administration  of  such  agents  as  ether  and  chloro- 
form, is  due  to  the  fact  that  the  dissolution  of  the  fatty 
material  in  nerve  tissue  induces  a  retraction  of  these  fila- 
ments, preventing  thereby  the  sensory  impulses  from  reach- 
ing their  respective  afferent  paths. 

The  Formation  of  Nerves  and  Their  Distal  and  Central 
End-organs. — At  a  short  distance  from  the  cell-body  the 
axone  acquires  a  thick  covering  which  is  known  as  the 


ss=sr 


FIG.  41. — Medullated    nerve    fiber,    showing    nodes    of    Ranvier;  X  660 
times.      (Schafer.) 


medullary  sheath.  Outside  of  this  one  lies  a  thin  membran- 
ous investment  which  is  known  as  the  neurilemma.  An 
axone  or  axis  cylinder  with  its  coverings  constitutes  a  nerve 
fiber  (Fig.  41).  Many  of  these  bound  together  into  bundles, 
form  a  nerve.  Accordingly,  the  cross-section  of  a  nerve 
presents  a  membranous  investment  and  numerous  partitions 
of  connective  tissue,  in  the  different  spaces  of  which  lie  the 
individual  nerve  fibers.  A  nerve,  such  as  the  sciatic,  em- 
braces something  like  300,000  fibers,  each  fiber  attaining 
a  diameter  of  about  10/x.  This  nerve,  therefore,  may  be 
compared  to  a  cable  consisting  of  a  large  number  of  wires. 

7 


98  MUSCLE    AND    NERVE 

A  collection  of  many  cell-bodies  within  the  central  nervous 
system  constitutes  a  nucleus.  Thus,  those  cell-bodies  which 
give  rise  to  the  axis  cylinders  of  the  vagus  nerve,  are  collec- 
tively designated  as  the  nucleus  of  the  vagus.  Similar  well 
defined  colonies  of  cell-bodies  within  or  without  the  central 
nervous  system  are  generally  called  ganglia.  The  spinal 
ganglion,  for  example,  embraces  a  large  number  of  cell- 
bodies,  the  sensory  axones  of  which  connect  peripheral  parts 
with  the  spinal  cord.  Whenever  these  collections  of  ganglion 


ep 


FIG.  42. — Transverse   section   of   a   nerve    (Median).     Ep,    epineurium; 
pe,  perineurium;  ed,  endoneurium.      (Landois  and  Stirling.) 

cells  control  a  definite  mechanism,  they  are  said  to  form  a 
center.  We  have  previously  noted  that  the  movements  of  the 
skeletal  musculature  are  controlled  by  a  group  of  large 
nerve  cells  which  are  situated  in  the  outer  layers  of  the 
cerebrum  next  to  the  fissure  of  Rolando.  Since  these  cells 
subserve  this  particular  function,  they  may  be  said  to  form 
the  motor  center  for  muscular  action.  Similar  groups  of 
ganglion  cells  are  found  in  different  parts  of  the  central 
nervous  system.  Our  attention  should  be  directed  at  this 
time  to  those  situated  in  the  upper  portion  of  the  spinal  cord 
or  medulla  oblongata.  These  cells  appear  in  three  principal 


THE    NERVE    IMPULSE    AND    REFLEX   ACTION 


99 


groups  which  severally  control  the  action  of  the  heart,  the 
caliber  of  the  bloodvessels,  and  the  respiratory  activity.  In 
other  words,  they  constitute  the  so-called  cardiac,  vasomotor 
and  respiratory  centers. 

If  we  now  follow  the  course  of  the  sciatic  nerve  to  distant 
parts,  we  find  that  its  individual  axones  finally  lose  their 
investing  sheaths  and  break  up  into  numerous  fine  filaments, 
each  terminating  in  a  structure  known  as  an  end-organ.  As 


FIG.  43. — End-plates;  chlorid  of  gold  preparation  to  show  the  axis  cylin- 
ders and  their  final  ramifications  of  fibrillae.      X  170.      (Szymonowicz.) 

has  been  emphasized  above,  the  sciatic  nerve  contains  efferent 
as  well  as  afferent  fibers.  The  former  convey  impulses  into 
the  end-organs,  and  the  latter  reversely  into  its  nucleus  or 
center.  It  is  to  be  noted  especially  that  the  end-organs  of 
the  motor  fibers  differ  very  markedly  in  their  structure  from 
those  attached  to  the  sensory  fibers.  To  simplify  matters, 
let  us  suppose  that  a  mechanical  impact  is  brought  to  bear 
upon  the  skin  of  the  foot  which  eventually  gives  rise  to  a 
contraction  of  the  muscles  of  this  part.  This  impact  is 
received  by  a  specialized  sense-organ  which  is  known  as  a 


100  MUSCLE    AND    NERVE 

receptor,  and  is  then  conveyed  in  the  form  of  an  afferent  im- 
pulse to  the  center.  Here  it  gives  rise  to  an  efferent  impulse 
which  is  conducted  outward  into  the  motor  end-organ  or 
effector,  activating  the  corresponding  muscle  fibers  (Fig.  40). 
These  receptors  and  effectors  possess  a  very  characteristic 
structure.  Ordinary  impacts  of  touch  are  received  by  the 
so-called  tactile  corpuscles  of  the  deeper  layer  of  the  skin. 
These  particular  receptors  appear  as  a  rule  as  bulbular 
enlargements  of  cells,  the  core  of  which  is  occupied  by  the 
terminal  nerve  fiber.  If  we  observe  for  a  moment  the  tactile 
corpuscle  illustrated  in  Fig.  44,  it  will  be  apparent  that  the 


FIG.  44. — Corpuscles  of  Grandry  from  the  duck's  tongue.  (I zguierdo .) 
A,  compound  of  three  cells,  with  two  interposed  discs,  into  which 
the  axis  cylinder  of  the  nerve,  n,  is  observed  to  pass;  in  B  there  is  but 
one  tactile  disc  enclosed  between  two  tactile  cells. 

displacement  suffered  by  the  skin  in  consequence  of  the 
impact  must  lead  to  a  similar  change  in  this  receptor  and  a 
mechanical  excitation  of  the  nerve  filament  in  its  interior. 
An  afferent  impulse  is  the  result  of  this  stimulation. 

The  effector  in  skeletal  muscle  tissue  presents  itself  as  a 
bulbular  ramification  of  nerve  fibrils  upon  the  sheath  of  the 
muscle  cell.  The  term  motor-plate  is  usually  employed 
when  reference  is  made  to  this  formation.  It  is  not  to  be 
supposed,  however,  that  a  muscle  composed  of  say  100,000 
cells,  is  supplied  by  an  equal  number  of  nerve  fibers.  Since 
the  axis  cylinder  of  each  fiber  divides  into  a  large  number  of 
fibrils,  each  of  which  ramifies,  one  axone  may  supply  as  many 
as  one  hundred  of  these  structures. 

It  is  to  be  remembered,  however,  that  we  have  described 
at  this  time  merely  one  particular  kind  of  receptor.  Many 
others  could  still  be  mentioned,  for  example,  that  of  the 
retina  of  the  eye  which  receives  the  ethereal  impacts  of 


THE   NERVE    IMPULSE    AND    REFLEX   ACTION  101 

light,  the  organ  of  Corti  of  the  internal  ear  through  which  the 
sound  waves  gain  psychic  recognition,  the  taste-buds  which 
receive  the  chemical  stimuli  giving  rise  to  the  sensation  of 
taste,  and  the  olfactory  cells  which  interact  Ayith  the  odori- 
ferous particles  of  the  air  to  j$bcltic6J  the  sensation  of  smell. 
The  motor  end-organ  of  smopth  mu3cle,p03S^es  a  structure 
similar  to  that  found  in  skeletal  rtfus'cte.J  '' 

The  Physiological  Properties  of  a  Nerve. — Under  normal 
circumstances  an  efferent  neurone  is  activated  through  its 
center,  and  an  afferent  neurone  through  its  receptor.  Under 
experimental  conditions,  however,  it  is  possible  to  stimulate 
a  neurone  at  any  point  of  its  course.  Consequently,  inas- 
much as  a  nerve  represents  a  collection  of  axones,  its  sole 
function  must  be  to  conduct  impulses  away  from  the  area 
in  which  they  are  generated.  The  element  chiefly  concerned 
with  this  function  is  the  axone  or  axis  cylinder.  Neither  the 
medullary  substance  nor  the  neurilemma  appear  to  be 
directly  involved  in  this  process  of  conduction,  because  many 
nerve  fibers  fulfill  their  function  perfectly,  although  not  in 
possession  of  a  medullary  sheath  nor  neurilemma.  While 
this  fact  has  been  clearly  established,  the  function  of  these 
envelopes  has  not  been  definitely  ascertained  as  yet.  But, 
inasmuch  as  a  nerve  fiber  presents  the  essential  characteris- 
tics of  an  insulated  wire,  it  has  been  supposed  that  the  medul- 
lary substance  is  insulatory  and  protective.  This  contention, 
however,  cannot  be  substantiated,  because  the  fibers  of  the 
sympathetic  system  are  all  non-medullated.  Neither  is  it 
correct  to  ascribe  to  it  a  nutritive  function,  because  the  axis 
cylinder  receives  its  nourishment  directly  from  the  cell- 
body  by  a  process  which  may  be  likened  to  protoplasmic 
streaming. 

Nervous  tissue  possesses  a  high  degree  of  irritability  and 
conductivity.  For  this  reason  it  is  frequently  referred  to  as 
the  master  tissue  of  the  body.  When  its  component  cells 
are  stimulated,  it  yields  a  peculiar  wave  of  excitation  or 
irritability  which  traverses  its  different  conductile  segments 
and  is  here  clearly  recognized  as  a  variation  in  their  elec- 
trical potential.  This  wave  constitutes  the  nerve  impulse. 
The  only  indication  of  the  activity  of  a  nerve  or  of  the 


102  MUSCLE   AND    NERVE 

passage  of  a  nerve  impulse  is  presented  by  this  electrical 
variation. 

Probably  the  first  idea  that  one  might  form  regarding  the 
nature  of  the'herve  impulse,  embodies  the  principles  of  the 
conduction  of  ah  electrical  wave  through  a  metal  conductor. 
Electricity  is.  not  matter,  but  merely  a  form  of  energy.  It 
does  not  acfuall^pass^  onward  like  water  through  a  tube,  but 
presents  itself  as  a  change  in  a  constant  physical  force.  It 
must  be  admitted  that  a  nerve  impulse  shows  similar  charac- 
teristics, but  since  its  speed  is  very  much  slower  than  that  of 
an  electrical  current  passed  through  a  copper  wire,  these 
processes  cannot  justly  be  said  to  be  identical.  Further- 
more, a  metal  conductor  may  convey  innumerable  electrical 
waves  without  showing  the  slightest  deterioration  or  destruc- 
tion of  its  substance.  To  a  certain  extent  this  is  also  true  of 
nerve-fibers,  because  they  do  not  exhibit  easily  recognizable 
signs  of  metabolism  ,and  fatigue.  When,  however,  a  nerve 
is  placed  in  an  atmosphere  deficient  in  oxygen,  it  soon  ceases 
to  conduct  impulses.  Lastly,  it  is  to  be  noted  that  it  liberates 
carbon  dioxid  when  stimulated. 

These  observations  strongly  suggest  that  conduction  in 
nerve  is  associated  with  definite  catabolic  changes.  Hence, 
since  catabolic  processes  must  always  be  followed  by  anabolic 
processes  in  order  to  prevent  cellular  exhaustion,  it  may  be 
surmised  that  a  certain  period  of  time  must  always  elapse 
between  the  successive  states  of  activity  of  the  nerve  during 
which  it  can  rebuild  its  substance.  The  fact  that  such 
changes  actually  occur  in  nerve  tissue,  is  substantiated  by 
the  observation  that  quickly  repeated  stimuli  eventually 
fail  to  excite  impulses.  Nerve  fibers,  therefore,  possess  a 
definite  refractory  period  during  which  they  are  impermeable 
to  stimuli,  but  this  period  is  extremely  short,  amounting  to 
only  about  0.006  of  a  second. 

It  must  be  granted,  however,  that  the  activity  of  nerve 
does  not  lead  to  an  appreciable  destruction  of  its  substance, 
and  that  whatever  dissimilation  takes  place  is  quickly 
remedied.  The  briefness  of  the  refractory  period  of  nerve 
fully  proves  this  statement.  Contrariwise,  the  cell-bodies 
and  end-organs  reveal  a  very  intense  metabolism  and  can, 


THE    NERVE    IMPULSE    AND    REFLEX    ACTION  103 

therefore,  be  fatigued  with  much  greater  ease.  It  has  pre- 
viously been  shown  that  an  excised  muscle,  if  stimulated 
excessively,  soon  loses  its  irritability  and  contractility.  A 
condition  of  this  kind,  however,  cannot  be  reproduced  in  the 
normal  body,  because  its  muscles  are  protected  against 
complete  fatigue  by  the  end-plates.  The  latter  are  easily 
fatigued,  and  cease  at  this  time  to  transfer  the  impulses  to 
the  muscle  substance.  On  account  of  their  greater  vulner- 
ability they  serve,  so  to  speak,  as  safety-valves  for  the  muscle. 

These  data  clearly  prove  that  the  basis  of  the  nerve  impulse 
is  formed  by  definite  chemical  changes.  It  is  true,  however, 
that  the  latter  are  associated  with  an  electrical  variation 
which  may  be  detected  with  the  aid  of  almost  any  suitable 
indicator,  such  as  a  galvanometer.  If  an  instrument  of  this 
kind  is  connected  with  a  nerve,  the  stimulation  of  the  latter 
gives  rise  to  very  characteristic  deflections  of  its  recording 
needle.  This  is  the  method  usually  employed  to  prove  that 
nerve  fibers  are  actually  conductile,  although  their  func- 
tional power  may  also  be  tested  by  observing  whether  their 
excitation  elicits  motor  effects,  such  as  the  contraction 
of  a  muscle.  Consequently,  a  nerve  impulse  may  be  defined 
as  a  metabolic  wave  in  neuroplasm  which  is  accompanied 
by  an  electrical  variation.  It  has  been  ascertained  that 
this  wave  progresses  in  the  nerves  of  warm-blooded  animals 
with  a  speed  varying  between  100  and  125  m.  in  a  second. 

The  preceding  definition  immediately  disposes  of  the 
assumption  that  the  nerve  impulse  is  a  pulsation  in  neuro- 
plasm, such  as  may  be  produced  in  a  rubber  tube  filled  with 
water  by  sharply  tapping  upon  its  wall.  Nerves  are  not 
hollow  tubes,  nor  is  neuroplasm  a  fluid.  Neither  is  it  per- 
missible to  liken  nerves  to  irritable  strings  of  tissue,  and  to 
state  that  a  nerve  impulse  results  whenever  these  sensitive 
fibers  are  pulled  upon.  To  be  sure,  we  frequently  character- 
ize certain  sensations  as  " nerve- tension "  and  "  nerve-strains," 
but  these  sensory  manifestations  have  their  origin  in  the 
receptors  of  the  skeletal  muscles  and  not  in  the  nerves 
themselves. 

Reflex  Action. — If  an  amoeba  or  other  unicellular  organism 
is  touched,  certain  specialized  reactions  ensue  in  its  substance 


104  MUSCLE    AND    NERVE 

in  consequence  of  this  impact  which  will  carry  it  away  from 
the  seat  of  the  stimulation.  Likewise,  if  a  mechanical  stimu- 
lus of  sufficient  strength  is  applied  to  the  skin  of  one  of  our 
hands,  the  muscles  of  this  part  contract,  thereby  giving  rise 
to  a  particular  movement.  The  principle  involved  in  these 
reactions  is  essentially  the  same,  because  the  stimulus  is 
followed  in  each  case  by  a  motor  action.  It  is  to  be  noted, 
however,  that  the  reactions  in  the  amoeba  are  accomplished 
without  nervous  tissue,  while  those  in  us  require  the  presence 
of  this  tissue. 

The  Animal  Kingdom  embraces  certain  types  of  organisms 
which  are  in  possession  of  nervous  elements  and  certain 
types  which  are  not.  The  former  may  again  be  divided 
into  two  groups,  this  division  being  based  upon  the  structural 
and  functional  development  or  complexity  of  this  tissue. 
All  the  higher  animals  are  equipped  with  ganglion  cells 
which  subserve  volition  and  other  psychic  manifestations. 
For  this  reason,  they  are  able  to  control  their  motor  responses 
by  volition,  while  the  lower  forms  are  not.  In  accordance 
with  this  general  outline,  the  motor  reactions  in  the  animal 
world  may  be  arranged  in  three  groups,  namely  as: 

(a)  reflex-like  actions,  or  reactions  effected  with  the  aid 
of  ordinary  protoplasm, 

(6)  reflex  actions,  or  reactions  accomplished  by  means  of 
nerve  tissue,  but  without  the  intervention  of  the  will,  and 

(c)  volitional  actions,  or  reactions  brought  about  with 
the  help  of  nervous  tissue  and  closely  controlled  by  the  will. 

The  higher  animal,  however,  also  presents  many  non-voli- 
tional reactions  or  reflexes,  in  addition  to  its  volitional  ones. 
Thus,  it  need  scarcely  be  mentioned  that  our  skeletal  muscu- 
lature is  under  the  control  of  the  will,  whereas  the  move- 
ments of  our  stomach,  intestine,  and  urinary  organs  are  not. 
Inasmuch  as  the  function  of  the  central  nervous  system  is 
more  difficult  to  understand  than  that  of  the  circulatory  or 
respiratory  system,  it  seems  advisable  to  postpone  the  discus- 
sion of  this  subject-matter  until  the  student  has  acquired  a 
knowledge  of  the  simpler  phases  of  physiology.  Reflex 
action,  on  the  other  hand,  plays  a  most  important  part  in 
the  production  of  all  motor  responses  and,  hence,  it  seems 


THE    NERVE    IMPULSE    AND    REFLEX    ACTION  105 

essential  that  the  student  obtain  a  clear  idea  regarding  its 
possibilities  before  endeavoring  to  analyze  the  phenomena 
connected  with  the  circulation  of  the  blood,  the  movements 
of  respiration,  and  other  functions. 

We  have  seen  that  the  most  elementary  structural  unit 
of  the  nervous  system  is  the  neurone.  Quite  similarly,  it 
may  be  held  that  the  reflex  act  constitutes  the  most  element- 
ary functional  unit  of  this  system.  It  is  an  act  executed  in 
response  to  a  stimulus  without  attention  or  volition.  This 
definition  implies  that  the  stimulus  is  converted  into  a  reac- 
tion without  being  first  subjected  to  an  association  in  the 
higher  centers  of  the  cerebrum.  On  this  account,  the  reflex 
act  bears  a  close  resemblance  to  the  reaction  shown  by  simple 
masses  of  living  matter,  although  it  requires  conduction 
through  nervous  tissue. 

The  example  usually  given  to  illustrate  reflex  action  is  the 
following:  If  the  hand  of  a  person  is  accidentally  brought  in 
contact  with  a  hot  object,  it  is  instantaneously  withdrawn 
from  the  seat  of  the  stimulation.  It  may  then  be  held  that 
the  person  felt  pain  and  withdrew  the  hand  in  consequence 
thereof  by  the  volitional  contraction  of  certain  muscles. 
This  explanation,  however,  is  not  correct,  because  it  can 
easily  be  proved  that  the  response  was  completed  before 
the  pain  was  actually  perceived.  Hence,  the  psychic  centers 
could  not  have  influenced  this  act,  and  the  muscular  response 
could  not  have  been  evoked  volitionally. 

As  another  example  might  be  mentioned  the  constriction 
and  dilatation  of  the  pupil.  If  we  gaze  through  a  window, 
the  size  of  this  orifice  is  diminished  in  consequence  of  the 
contraction  of  that  layer  of  smooth  muscle  tissue  of  the  iris 
which  is  arranged  circularly  in  its  substance.  Contrari- 
wise, low  intensities  of  light  give  rise  to  a  contraction  of 
those  smooth  muscle  cells  which  permeate  it  radially  inward 
from  its  periphery.  The  edge  of  the  iris  is  then  retracted, 
and  the  size  of  the  pupil  increased.  It  need  scarcely  be 
mentioned  that  this  action  on  the  part  of  the  iris  varies  the 
size  of  the  bundle  of  light  which  is  permitted  to  enter  the 
interior  of  the  eye.  It  cannot  be  influenced  by  attention 
nor  volition  and  hence,  is  a  simple  reflex  act. 


106  MUSCLE   AND    NERVE 

Other  familiar  reflexes  are  the  acts  of  coughing  and  sneezing 
which  serve  to  protect  the  respiratory  passage  against  the 
entrance  of  foreign  bodies.  In  these  instances,  the  excita- 
tion of  the  lining  membrane  of  the  nose  or  pharynx  gives  rise 
to  a  forced  expiration  and  a  powerful  blast  of  air  which  is 
intended  to  remove  the  stimulus. 

In  order  to  be  able  to  obtain  these  elementary  nervous 
reactions  at  least  two  neurones  must  be  present:  namely,  a 
sensory  one  and  a  motor  one.  Their  union  is  effected  in  the 
synapse.  The  neurone  which  conveys  the  impulse  from  the 
receptor  to  the  center,  constitutes  the  afferent  path  of  this 
circuit,  and  the  one  conducting  the  impulse  from  the  center 
to  the  effector,  its  efferent  path.  Stated  in  detail,  a  simple 
reflex  system  invariably  embraces  a  receptor,  an  afferent 
path,  a  center,  an  efferent  path,  and  an  effector. 


PART  II 

THE  CIRCULATION  OF  THE  BLOOD  AND 
LYMPH 

CHAPTER  X 
THE  LYMPH 

General  Characteristics  of  the  Body-fluids. — Every  cell 
contains  a  certain  quantity  of  water  and  a  certain  amount 
of  solids.  This  is  true  of  free-living  cells  as  well  as  of  those 
constituting  the  different  tissues  and  organs  of  the  higher 
animals.  Roughly  speaking,  it  may  be  said  that  the  rela- 
tionship between  these  constituents  is  as  3 : 1,  i.e.,  three  parts 
of  water  and  one  part  of  solids.  It  is  true,  however,  that 
cells  differ  somewhat  in  this  respect,  showing  a  water  con- 
tent of  65  to  79  per  cent,  in  the  different  tissues.  As  might 
be  expected,  the  blood  is  very  rich  in  water.  It  contains 
only  21  parts  pf  dry  solids,  and  of  these  almost  12  parts  are 
apportioned  to  its  corpuscular  elements. 

In  the  lowest  organisms  specialized  circulatory  fluids, 
such  as  the  blood  and  lymph,  are  not  present.  The  sub- 
stance of  these  forms  is  permeated  by  a  fluid  which  is  com- 
posed chiefly  of  water  and  is  designated  as  the  body-fluid. 
The  composition  of  this  fluid  is  not  very  different  from  that 
of  the  surrounding  medium,  because  it  is  separated  from 
the  latter  by  only  a  delicate  membrane  which  permits  free 
osmotic  interchanges.  The  nutritive  substances  and  respira- 
tory gases  diffuse  through  this  membrane  in  either  direction. 

The  body-fluid  is  able  to  acquire  a  higher  concentration 
and  differentiation  only  when  these  interchanges  with  the 
outside  medium  are  somewhat  restricted.  This  stage  of 
development  is  attained  by  those  organisms  which,  on 

107 


108         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

account  of  their  size  and  complexity,  do  not  interchange 
solely  through  their  enveloping  membrane,  but  principally 
through  a  membranous  tube  which  traverses  the  entire 
body  and  is  known  as  the  celom  or  alimentary  canal.  This 
channel  is- beset  with  many  smaller  tubules  and  recesses  which 
extend  into  the  outlying  colonies  of  cells  and  bring  the  latter 
into  closer  relation  with  their  sources  of  nutritive  supply.  In 
the  animals  of  somewhat  greater  complexity,  these  recesses 
are  separated  from  the  main  body-cavity  and  form  a  series 
of  spaces  containing  a  fluid  which,  on  account  of  its  greater 
isolation,  is  able  to  acquire  and  retain  a  much  more  char- 
acteristic composition.  These  elementary  spaces  form  the 
beginning  of  the  circulatory  system  of  the  higher  animals. 

The  fluid  occupying  the  intercellular  spaces  of  the  lowest 
forms,  is  shifted  in  consequence  of  the  general  movements 
of  the  organism.  In  the  higher  forms,  on  the  other  hand, 
it  is  propelled  through  an  extensive  system  of  separate 
channels  by  means  of  a  highly  specialized  pumping  organ 
which  is  known  as  the  heart.  Because  of  its  imparted 
motion,  the  blood  is  in  the  best  possible  position  to  play  the 
role  of  a  middleman  whose  duty  it  is  to  convey  nutritive 
particles  to  the  distant  cells  and  to  relieve  the  latter  of  their 
waste  products.  We  shall  see  later  that  it  is  able  to  fulfill 
this  function  very  efficiently,  because  it  comprises  different 
cellular  elements,  each  of  which  has  a  particular  purpose 
to  fulfill. 

Another  very  excellent  provision  is  that  all  the  higher 
animals  are  equipped  with  two  types  of  body-fluids,  which 
are  severally  designated  as  the  blood  and  lymph.  The 
former  is  a  much  more  complex  medium  than  the  latter, 
because  it  contains  in  addition  to  the  plasma  a  large  number 
of  separate  cells  of  different  size  and  shape.  The  arrange- 
ment usually  followed  in  the  construction  of  a  tissue  is  this: 
Every  cell  is  surrounded  by  intercellular  material  which 
possesses  a  lesser  density  than  its  cytoplasm  and  nuclear 
material.  For  this  reason,  it  is  commonly  stated  that  the 
different  tissue  cells  are  bathed  in  lymph.  This  fluid  con- 
sists of  a  large  amount  of  water  in  which  the  different  sub- 
stances required  by  the  cells  are  dissolved.  As  far  as  its 


THE    LYMPH  109 

general  character  is  concerned,  it  approaches,  therefore,  most 
closely  the  ordinary  body-fluid  of  the  lowest  forms.  Further- 
more, since  the  cells  constantly  interchange  materials  with 
the  blood  through  this  particular  medium,  it  forms  an  im- 
portant functional  link  between  these  two  systems. 

The  interposition  between  the  complex  blood  and  the  cells 
of  so  simple  a  medium  as  the  lymph  is  of  the  greatest 
economic  value.  It  must  be  evident  that  a  much  greater 
number  of  blood-channels  and  a  very  much  larger  quantity 
of  blood  would  be  required  if  this  provision  had  not  been 
made.  Clearly,  the  lymph  furnishes  a  very  simple  means 
of  bringing  every  cell  into  direct  functional  relation  with  the 
blood.  Owing  to  its  wide  distribution,  it  is  possible  to 
restrict  the  quantity  of  the  blood  without  endangering  the 
life  of  the  cells. 

The  Formation  of  the  Lymph. — The  channels  of  the  blood 
vascular  system  eventually  divide  into  a  multitude  of  very 
small  tubules  which  are  known  as  capillaries.  These  finest 
ramifications  of  the  vascular  system  are  limited  by  a  wall  of 
very  thin,  plate-like  cells  of  endothelium.  External  to  this 
lining  are  situated  the  different  cells  of  the  tissues.  A  thin 
layer  of  lymph  is  interposed  between  the  latter  and  the 
walls  of  the  capillary.  As  the  blood  traverses  these  small 
tubules,  a  portion  of  its  fluid  part  or  plasma  is  transferred 
into  the  intercellular  lymph-spaces,  but  since  its  formed 
elements  as  well  as  larger  nutritive  elements  cannot  get 
through,  the  lymph  is  really  diluted  plasma.  It  is  to  be 
remembered,  however,  that  its  formation  is  not  due  solely  to 
mechanical  factors,  such  as  differences  in  pressures,  but  also 
to  osmosis,  diffusion,  and  certain  chemico-physical  activities 
of  the  lining  cells,  the  nature  of  which  is  as  yet  not  fully 
recognized. 

It  may  then  be  asked  whether  this  portion  of  the  blood 
plasma  which  has  been  diverted  into  the  spaces  between  the 
cells,  is  actually  lost  to  the  blood.  A  glance  at  Fig.  45  will 
show  that  it  is  not,  because  the  lymph  spaces  are  united 
into  very  delicate  tubules  and  these  in  turn  into  ducts,  until 
eventually  a  main  collecting  tube  has  been  formed  which 
opens  directly  into  one  of  the  central  veins.  Having  in  this 


110        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

way  regained  the  blood  vascular  system,  the  lymph  is  again 
forced  into  the  peripheral  vessels  as  a  part  of  the  plasma. 

It  will  be  seen,  therefore,  that  the  tissues  of  our  body  are 
permeated  by  two  sets  of  tubules :  namely,  those  belonging  to 
the  blood  vascular  system  and  those  constituting  the  lymph- 
atic system.  If  the  surface  of  the  skin  is  slightly  scraped,  a 
number  of  fine  droplets  of  a  watery  fluid  will  be  seen  to 


FIG.  45. — Scheme  of  the  circulatory  system  of  the  lymph.  C,  blood 
capillaries;  T,  tissue  cells ;  L. D,  lymphatics;  L,  lacteals;  V,  villi  of  intes- 
tine: Li,  liver;  R,  receptacle;  T.D,  thoracic  duct;  S.v,  subclavian  vein. 

escape  from  the  opened  lymph  ducts.  These  droplets  of 
lymph  finally  coalesce  and  coagulate,  forming  a  thin  film 
over  the  injured  area.  But,  if  the  skin  is  incised  more  deeply, 
a  certain  number  of  blood-capillaries  will  also  be  opened 
which  discharge  their  contents  into  the  extravasate,  impart- 
ing to  it  a  distinct  reddish  color.  Since  the  blood  also 
possesses  the  power  of  clotting,  a  common  coagulum  will 
eventually  result. 

The  Distribution  of  the  Lymphatic  Channels. — The  walls 
of  the  finest  lymphatic  tubules  are  composed  of  thin,  plate-like 
cells  and  present,  therefore,  a  structure  very  similar  to  that 
of  the  blood-capillaries.  The  larger  ducts,  on  the  other 
hand,  are  strengthened  by  a  certain  amount  of  connective 


THE   LYMPH  111 

tissue,  and  acquire  a  diameter  of  several  millimeters.  It  is 
also  of  interest  to  note  that  the  orifices  of  the  smaller  lymph- 
atics are  guarded  by  valves  which  usually  consist  of  two  cup- 
shaped  flaps  directed  toward  the  larger  tubule.  They  close 
immediately  if  the  pressure  in  the  more  central  lymphatic 
rises  above  that  prevailing  in  the  more  distal  channel.  In 
this  way  a  back  flow  is  made  practically  impossible. 

The  lymphatic  channels  are  beset  at  frequent  intervals 
with  rounded  glandular  structures,  the  so-called  lymph-nodes. 
Small  amoeboid  cells  are  formed  in  these  nodular  bodies  which 
are  known  as  lymph-corpuscles  or  lymphocytes.  These  cells 


FIG.  45a. — Cross-section    of   lymphatic    vessel    to    show  arrangement  of 

valves. 

are  flushed  into  the  bloodstream,  where  they  change  into  the 
white  cells  of  the  blood  and  may  also  be  transferred  into 
leukocytes.  The  lymphatic  glands  also  play  the  part  of 
filters,  retaining  any  foreign  substance  that  may  have  entered 
the  lymph-stream.  For  this  reason,  it  is  frequently  noted 
that  micro-organisms  are  held  back  in  these  sieve-like  struc- 
tures, evoking  here  an  inflammatory  reaction  and  causing 
them  to  become  enlarged  and  painful  to  the  touch.  In  many 
cases,  a  general  inflammation  of  the  lymphatics  then  results, 
causing  them  to  become  sharply  outlined  against  the  integu- 
ment as  delicate,  bright  red  threads  which  extend  from  the 
infected  area  toward  the  next  colony  of  lymphatic  glands. 
If  the  infectious  material  is  particularly  virulent,  it  may 
succeed  in  getting  beyond  these  peripheral  stations,  but 
may  again  be  effectively  blocked  by  the  glands  situated 
farther  centrally.  A  general  infection  of  the  body  results 
only  if  all  these  stations  have  been  passed  successively. 

As  has  been  stated  above,  the  main  collecting  channels  dis- 
charge their  contents  directly  into  the  venous  bloodstream. 
The  largest  of  these  is  the  thoracic  duct  which  joins  the 
venous  system  at  the  point  of  confluence  of  the  external 


112 


THE  CIRCULATION  OF  THE  BLOOD  AND  LYMPH 


jugular  and  left  subclavian  veins,  and  drains  the  legs,  ab- 
dominal organs,  and  entire  left  side  and  lower  half  of  the  right 
side  of  the  chest.  In  man  it  is  38  to  45  cm.  in  length  and  of 


FIG.  46. — Lymph  nodes  and  lymphatic  drainage  of  the  upper  extrem- 
ity,   mammary    gland    and   thoracic  wall;   RLD,   right  lymphatic  duct.    - 
(Radasch.) 

about  the  size  of  a  goose  quill.  It  lies  to  the  left  of  the  spinal 
column  and  thoracic  aorta.  A  separate  duct,  about  2.5 
cm.  in  length,  is  provided  for  the  upper  right  side  of  the 
chest.  Separate  channels  which  are  known  as  the  right 


THE   LYMPH 


113 


and  left  cervical  lymphatics,  return  the  lymph  from  the 
neck  and  head. 

On  account   of  their  small  caliber  and  watery  appear- 
ance of  their  contents,  the  distal  lymphatics  cannot  be  made 


B 


FIG.  47. — The  distribution  of  the  lymphatics.  A,  The  domain  of  the 
thoracic  duct  (unshaded  portion);  B,  right  lymph  duct;  C,  left  cervical 
duct;  D,  right  cervical  duct. 

out  with  great  ease,  while  those  of  the  intestine  are  very 
sharply  outlined  whenever  fat  is  being  absorbed.  It  will 
be  shown  later  that  by  far  the  largest  portion  of  the  digested 
fat  attains  the  blood  through  the  so-called  lacteals  which  form 
the  finest  radicles  of  the  lymphatic  system  in  this  organ. 


114        THE   CIRCULATION    OF   THE   BLOOD    AND   LYMPH 

These  lacteals  are  situated  within  small  finger-like  projections 
of  the  mucosa  which  are  known  as  villi.  After  the  fine  glob- 
ules of  fat  have  entered  these  tubules,  they  impart  a  milky 
appearance  to  the  lymph  contained  therein,  so  that  each 
lymphatic  may  readily  be  traced  from  its  very  beginning  to 
its  point  of  confluence  with  the  thoracic  duct.  Lymph 
containing  large  amounts  of  fat  is  known  as  chyle. 

The  Flow  of  the  Lymph. — The  factors  which  may  be  held 
responsible  for  the  return  of  the  lymph,  are : 

(a)  Differences  in  Pressure. — The  lymph  is  formed  under 
the  pressure  prevailing  in  the  blood-capillaries.     It  amounts 
to  about  40  mm.  Hg.     In  the  venous  system,  on  the  other 
hand,  the  pressure  is  below  that  of  the  atmosphere,  and 
amounts  to  about  —5  mm.  Hg.     Accordingly,  the  pressure 
throughout  the  lymphatic  system  decreases  by  about  50 
mm.  Hg,  a  difference  equal  to  that  which  causes  the  venous 
blood  to  be  returned  to  the  heart. 

(b)  Muscular  Movements. — Every  contraction  of  our  skele- 
tal muscles  exerts  a  mechanical  influence  upon  the  extremely 
soft  walls  of  the  lymphatics,  pressing  their  contents  onward. 

(c)  Lymphatic   Valves. — Inasmuch  as  these  membranous 
flaps  open  only  in  the  direction  of  the  veins,  a  backward  flow 
of  the  lymph  cannot  take  place. 

(d)  Lymph-hearts. — The  lymphatics  of  certain  animals  are 
beset  with  pulsating  organs  which  act  in  the  manner  of  the 
blood-heart   and   establish   definite   pressures   within   their 
chambers.     The  orifices  of  the  lymphatics  leading  into  these 
secondary  hearts  are  guarded  by  valves  and  are  tightly 
closed  during  the  period  of  contraction  of  these  organs. 
They    open    when    relaxation    sets    in.     Contrariwise,    the 
valve  guarding  the  orifice  of  the  collecting  lymphatic  opens 
during  the  phase  of  contraction  and  closes  during  the  phase 
of   relaxation.     By   this   means   the   lymph   is   constantly 
forced  onward  in  the  direction  of  the  veins. 

(e)  Suction-action. — Inasmuch  as  the  principal  lymphatic 
enters  the  vein  at  almost  a  right  angle,  the,  lymph  is  actually 
drawn  into  the  venous  channel. 


CHAPTER  XI 


THE  BLOOD 

Composition  and  General  Characteristics. — The  blood 
is  a  viscous  fluid,  consisting  of  a  watery  portion  or  plasma 
and  various  solids.  The  latter  are  held  in  solution  as  well 
as  suspension,  and  embrace  different  nutritive  substances 
and  a  large  number  of  formed  elements  which  are  known  as 
corpuscles.  The  corpuscular  portion  consists  of  red  cells 
(erythrocytes) ,  white  cells  (leukocytes),  and  platelets  (throm- 
bocytes). 


Blood 


Water 


Solids 


Plasma 


dissolved  nutritive  substances 
larger  nutritive  particles 

f  red,  erythrocytes 
Corpuscles  <  white,  leukocytes 

(  platelets,  thrombocytes 

When  present  in  larger  amounts,  the  blood  of  the  mammals 
exhibits  a  very  characteristic  color,  varying  between  scarlet 
red  and  purple.  This  difference  is  due  to  the  fact  that  the 
amount  of  oxygen  contained  in  the  pigmentous  material 
of  the  red  cells  is  greatest  in  the  arterial  blood  and  least  in  the 
venous  blood.  It  is  evident,  therefore,  that  the  color  of  the 
blood  must  be  of  distinct  value  to  the  physician,  because  it 
betrays  the  degree  of  oxygenation  of  the  tissues.  A  low 
content  in  oxygen  invariably  suggests  itself  by  a  bluish 
color  of  the  exposed  parts,  such  as  the  lips  and  skin  of  the 
face.  Single  red  corpuscles,  however,  possess  a  yellowish 
color,  and  a  distinct  reddish  hue  is  imparted  to  the  blood 
only  when  these  elements  are  present  in  considerable  numbers. 

115 


116        THE    CIRCULATION    OF   THE  BLOOD   AND    LYMPH 

Blood  possesses  a  salty  taste  and  faint  odor.  Its  specific 
gravity  is  about  1.055.  Its  temperature  varies  in  different 
parts  of  the  body,  being  highest  in  those  vessels  which  are 
well  protected  by  the  tissues,  and  lowest  in  those  situated 
near  the  surface.  Thus,  the  blood  in  the  veins  of  the  liver 
may  attain  a  temperature  of  39.7°  C.  (103°  F.),  while  that  in 
the  fingers,  nose,  and  cheeks  may  possess  a  temperature  of 
only  36.0°  C.  (97.7°  F.).  Naturally,  the  blood  receives  its 
heat  from  the  cells  of  the  different  tissues  and  transfers  it 
later  on  to  the  air  either  by  radiation  or  in  the  form  of  bound 
heat.  The  source  of  this  heat,  therefore,  lies  in  the  oxidations 
of  the  tissue  cells. 

The  Distribution  and  Total  Quantity  of  the  Blood. — The 
total  quantity  of  blood  present  in  an  animal  may  be  measured 
in  different  ways.  The  method  most  easily  followed  is  the 
one  which  purposes  to  collect  all  the  blood  by  simply  opening 
a  large  artery  and  permitting  its  contents  to  flow  into  a 
graduated  receptacle.  This  procedure,  however,  is  by  no 
means  exact,  because  some  of  the  blood  always  remains  in 
the  veins  and  capillaries  and  cannot  be  expelled  by  the  force 
of  the  heart  beat.  Much  more  valuable  data  have  been 
obtained  by  the  chemical  and  colorimetric  methods.  While 
these  procedures  cannot  be  described  in  detail  at  this  time, 
it  may  be  of  interest  to  note  the  values  which  have  been 
ascertained  with  their  aid.  The  earlier  determinations  have 
placed  the  blood-volume  of  the  human  subject  at  about 
5  liters.  More  recently,  however,  it  has  been  found  that 
any  calculation  made  upon  the  basis  of  ^3  of  the  body- 
weight,  is  too  high,  and  that  a  volume  of  about  4  liters  or 
9  pounds  is  much  nearer  the  correct  value.  This  statement 
implies  that  the  total  quantity  of  the  blood  amounts  to  M?~ 
3^20  °f  the  body- weight.  The  latter  figure  should  be  em- 
ployed for  these  calculations  if  the  weight  of  the  person  is 
augmented  by  a  heavy  deposition  of  fatty  tissue. 

After  the  blood  has  been  ejected  from  the  heart  it  is  dis- 
tributed to  the  different  organs  of  the  body  in  amounts  cor- 
responding to  their  activity.  Obviously,  the  bones,  tendons, 
and  cartilages  require  only  a  very  small  amount  of  nutritive 
material,  because  they  do  not  undergo  intense  metabolic 


THE  BLOOD  117 

changes  after  they  have  attained  their  full  development. 
Glandular  tissues,  on  the  other  hand,  are  very  active  and 
must,  therefore,  be  in  possession  of  a  constant  supply  of 
material  from  which  their  products  can  be  formed.  Accord- 
ingly, it  is  found  that  such  organs  as  the  kidneys,  brain,  and 
glands  of  internal  secretion  are  in  constant  need  of  copious 
amounts  of  blood.  The  thyroid,  for  example,  receives  560 
c.c.  of  blood  in  a  minute  per  100  grams  of  substance,  while 
the  connective  tissue  structures  of  the  head  obtain  only  5 
c.c.  in  a  minute  for  the  same  weight  of  substance. 

If  we  would  suddenly  ligate  different  bloodvessels  so  as  to 
divide  the  body  into  several  separate  vascular  areas,  we 
would  find  that  one-quarter  of  the  total  amount  of  blood  is 
contained  at  any  one  time  in  the  heart,  lungs  and  large 
vessels,  one-quarter  in  the  liver  and  portal  vessels,  one-quarter 
in  the  skeletal  muscles,  and  one-quarter  in  the  remaining 
organs. 

The  Red  Corpuscles. — If  a  small  drop  of  human  blood  is 
collected  upon  a  glass  slide  and  is  placed  under  the  ocular 


FIG.  48a.— Human  red  corpuscle  FIG.  486. — Red  corpuscle  of  frog 

placed  flat  and  on  edge.  placed  flat  and  on  edge. 

of  a  microscope,  it  will  be  seen  to  contain  a  very  large  number 
of  cellular  elements,  among  which  the  red  corpuscles  or 
erythrocytes  are  the  most  conspicuous.  These  bodies 
appear  as  flattened,  circular  discs,  possessing  a  diameter  of 
7.6/i  0^200  of  an  inch),  and  a  thickness  of  about  2ju.  It  will 
be  noted  that  the  central  area  of  each  cell  is  much  thinner 
than  its  marginal  one  and  hence,  its  cross-section  must 
exhibit  the  general  outline  of  a  dumbbell.  In  seeking  an 
explanation  for  this  structural  peculiarity,  it  should  be 


118        THE    CIRCULATION    OF   THE  BLOOD    AND    LYMPH 

remembered  that  the  human  red  cell  loses  its  nucleus  very 
soon  after  its  formation  and  enters  the  circulatory  system 
non-nucleated. 

An  exception  to  this  rule  results  only  when  the  corpuscle- 
forming  organs  are  in  a  state  of  the  highest  possible  activity 
in  order  to  replace  cells  which  have  been  lost  by  bleeding  or 
other  destructive  processes.  In  this  connection  it  should 
also  be  mentioned  that  these  corpuscles  are  developed  during 
adult  life  in  the  red  marrow  of  the  bones.  Their  mother- 
cells  are  the  so-called  erythroblasts  of  the  red  marrow.  This 
fatty  material  occupies  the  extremities  of  the  long  bones, 
while  their  shafts  are  filled  with  yellow  marrow.  Whenever 
a  rapid  destruction  of  red  corpuscles  has  taken  place  in 
consequence  of  a  hemorrhage,  the  red  marrow  increases  in 
mass  at  the  expense  of  the  neighboring  yellow  marrow.  This 
change  is  also  well  illustrated  by  the  hibernating  animals 
which  undergo  very,  decisive  metabolic  changes  at  the  begin- 
ning of  spring,  and  again  late  in  the  autumn.  When  their 
increased  bodily  activity  suddenly  calls  for  a  greater  number 
of  red  corpuscles  to  take  care  of  their  extra  demand  for 
oxygen,  the  red  marrow  rapidly  increases  in  bulk.  The 
reverse  change  takes  place  when  the  metabolism  of  these 
animals  is  gradually  reduced  to  its  low  standard  of  the  winter 
months. 

Under  ordinary  circumstances,  the  human  red  corpuscles 
are  destroyed  in  constant  numbers.  They  become  senile 
while  circulating,  and  are  subsequently  broken  down  in  the 
liver,  their  remnants  being  here  employed  in  the  formation 
of  the  pigments  of  the  bile.  This  steady  destruction  must 
be  compensated  for  by  a  constant  formation. 

The  red  blood-corpuscles  of  the  cold-blooded  animals  are 
elliptical  in  shape  and  retain  a  very  conspicuous  nucleus 
throughout  their  life.  Those  of  the  frog  measure  25ju  in 
length  and  15ju  in  breadth,  and  are,  therefore,  more  than 
three  times  as  large  as  the  human  red  cell.  Still  larger 
corpuscles  are  found  in  the  salamanders,  where  they  attain  a 
length  of  about  75ju. 

The  number  of  the  red  corpuscles  varies  with  their  size. 
Thus,  the  frog  possesses  only  about  1,500,000  red  cells  in 


THE  BLOOD  119 

each  cubic  millimeter  of  blood,  whereas  human  blood 
contains  about  5,000,000  in  this  amount.  Accordingly, 
the  total  number  of  red  cells  present  in  a  person  weighing 
about  150  Ibs.  must  be  something  like  20  trillions.  In  this 
connection,  brief  reference  should  also  be  made  to  the  fact 
that  the  people  inhabiting  high  regions  possess  a  larger 
number  of  red  corpuscles  than  those  living  at  lower  levels. 

Each  red  cell  consists  of  a  delicate  saccule,  in  the  in- 
terior of  which  is  contained  a  substance  known  as  hemo- 
globin. In  reality,  therefore,  the  red  cell  may  be  regarded  as 
a  tiny  mass  of  hemoglobin  which  possesses  the  peculiar 
property  of  uniting  with  the  oxygen  of  the  atmospheric  air. 
By  virtue  of  its  affinity  for  this  gas,  this  constituent  confers 
upon  the  red  corpuscle  its  characteristic  function  of  an  oxy- 
gen carrier.  On  leaving  the  capillaries  of  the  lungs  each 
red  cell  is  fully  loaded  with  this  gas.  Its  hemoglobin  is  then 
in  the  form  of  oxy-hemoglobin,  i.e.,  completely  charged  with 
oxygen.  In  the  tissues,  a  part  of  this  gas  is  transferred 
to  the  cells  to  enable  them  to  oxidize  their  nutritive  sub- 
stances. Hemoglobin  so  partially  depleted  of  its  store  in 
oxygen,  is  called  reduced  hemoglobin.  These  differences 
are  responsible  for  the  variations  in  the  color  of  the  blood. 
The  arterial  blood  is  bright  red,  because  it  contains  large 
amounts  of  oxygen,  whereas  venous  blood  is  purplish,  because 
it  embraces  smaller  quantities  of  this  gas. 

In  accordance  with  the  foregoing  discussion,  it  may  be 
concluded  that  any  reduction  in  the  number  of  the  red  cor- 
puscles or  any  diminution  in  the  hemoglobin  content  of  the 
individual  red  cells  must  greatly  diminish  the  oxygen  carrying 
capacity  of  the  blood  and  finally  lead  to  an  improper  aeration 
of  the  tissues.  The  term  "anosmia"  signifies  that  the  blood 
contains  an  abnormally  small  number  of  red  cells,  possibly 
only  two  to  three  millions  to  the  cubic  millimeter.  This 
condition  may  have  its  cause  either  in  a  decreased  production 
or  an  increased  destruction  of  these  corpuscles.  As  causa- 
tive agents  of  it  might  be  mentioned:  an  excessive  loss 
of  blood,  circulating  toxins  and  poisons,  amceba  and  filiaria 
infections  of  the  intestines  and  blood,  and  a  general  unhygi- 
enic mode  of  life.  The  term  chlorosis  suggests  that  the  hemo- 


120        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 


globin  content  of  the  blood  is  below  normal.  This  condition 
is  usually  associated  with  anaemia,  because  a  decided  reduction 
in  the  number  of  the  red  cells  must  also  diminish  the  total 
amount  of  hemoglobin,  although  each  red  corpuscle  may 
still  be  in  possession  of  its  necessary  quantity  of  hemoglobin. 
It  may  also  happen  that  the  hemoglobin  content  of  each 
red  cell  is  lower  than  it  should  be,  although  the  total  number 
of  these  elements  is  practically  normal.  It  should  be  re- 
membered, however,  that  distinct  disturbances  arise  only 
when  the  hemoglobin  falls  below  80  per  cent,  of  normal,  and 
that  even  perfectly  normal  persons  rarely  show  a  hemoglobin 
content  of  100  per  cent. 


FIG.  49. — Different  varieties  of  human  white  corpuscles.  A,  lympho- 
cyte; B,  mononuclear  leukocyte;  C,  transitional  form;  D,  polynuclear 
leukocyte;  E,  eosinophile  leukocyte;  F,  mast-cell.  (After  Szymonowicz.) 

The  White  Corpuscles. — The  colorless  corpuscles  of  the 
blood  vary  greatly  in  size.  The  smaller  ones  possess  a 
diameter  of  only  4/x,  while  the  larger  ones  may  attain  a 
diameter  of  14/i.  Both  types  of  cells  are  irregular  in  outline, 
because  they  possess  amoeboid  powers  and  alter  their  con- 
tours constantly.  The  larger  ones,  which  are  known  as 
leukocytes,  exhibit  the  most  decided  changes.  When 


THE  BLOOD 


121 


placed  under  the  ocular  of  a  microscope,  yielding  a  magnifi- 
cation of  about  500  diameters,  they  may  be  seen  to  send  out 
delicate  processes  in  particular  directions,  meanwhile  re- 
tracting their  protoplasm  elsewhere.  In  this  way,  they  are 
able  to  proceed  from  place  to  place,  but,  naturally,  their 
movements  are  slow  and  require  minutes  for  their  completion. 
When  in  the  bloodstream,  these  cells  are  usually  found  in  the 
outer  clear  zone  of  the  plasma,  where  they  attach  themselves 
to  the  wall  of  the  vessel.  Every  now  and  then,  however, 
they  become  detached  and  roll  on  to  more  distant  parts  of 
the  vascular  system.  Their  number  varies  between  6000 
and  10,000  to  the  cubic  millimeter  of  blood  and,  hence,  their 
proportion  is  as  1 : 700  red  cells. 

These  amoeboid  qualities  are  responsible  for  the  power  of  the 
leukocytes  to  surround  and  destroy  foreign  particles  that 


FIG.  50. — Leukocytes  engulfing  particles  of  India  ink. 

may  have  entered  the  bloodstream.  This  process  which  is 
known  as  phagocytosis,  constitutes  one  of  the  most  important 
safeguards  of  the  body  against  invasion  by  bacteria.  In  this 
connection  it  should  also  be  noted  that  these  cells  are  able  to 
attach  themselves  firmly  to  the  wall  of  the  bloodvessel  and 
to  perforate  it  in  order  to  gain  access  to  the  neighboring 
tissues.  This  migration  is  the  natural  consequence  of  the 
invasion  of  the  body  by  pus  producing  bacteria.  Thus,  if  a 
colony  of  these  micro-organisms  has  succeeded  in  finding 
lodgment  in  the  deeper  layers  of  the  skin,  the  leukocytes  are 
attracted  to  them  in  a  chemical  way.  They  leave  the  blood- 
vessels and  enter  the  tissues,  where  they  engulf  and  digest 


122        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 


this  toxic  material.  Accordingly,  the  leukocytes  may  be 
said  to  serve  the  purposes  of  policemen,  because  they  are  in 
evidence  everywhere  as  safeguards  against  microbic  invasion. 
They  are  greatly  aided  in  the  performance  of  this  important 
function  by  certain  changes  in  the  character  of  the  blood- 
stream which  materially  facilitate  their  movements.  Thus, 
supposing  that  a  certain  region  of  the 
body  has  become  the  seat  of  an  infection, 
it  will  be  noticed  that: 

(a)  The  bloodvessels  of  this  part  relax, 
causing  the  speed  of  the  bloodflow  to  be 
diminished  and  the  total  quantity  of  the 
blood  to  be  increased.      This  change  is 
responsible  for  the  greater  warmth  and 
redness  of  the  inflamed  region. 

(b)  The  leukocytes  enter  the  vessels 
of  this  area  in  large  numbers,  attaching 
themselves    everywhere    to    their    walls. 
Having  finally  succeeded  in  perforating 
the  lining  cells,  they  migrate  into  the 
surrounding  tissues. 

(c)  The  leukocytes  engulf  and  devour 
the   bacteria.      During  this   interaction 
many  of  them  are  killed,  and  are  changed 
into    the    so-called    pus-corpuscles,    the 

principal  constituent  of  pus.  Inasmuch  as  an  extra  amount 
of  lymph  is  at  this  time  exuded  into  the  inflamed  tissues, 
their  volume  is  increased.  A  certain  pressure  is  then 
brought  to  bear  upon  the  sensory  nerve  filaments  situated 
among  their  cells  with  the  result  that  the  entire  area  becomes 
painful  to  the  touch.  It  will  be  noted,  therefore,  that  the 
four  cardinal  symptoms  of  an  inflammation  are:  redness, 
swelling,  heat,  and  pain. 

The  Blood  Platelets. — These  formed  elements  of  the  blood 
appear  as  irregular  masses  of  protoplasm,  possessing  a 
diameter  of  only  about  3/z.  They  consist  of  a  dark  center, 
representing  in  all  probability  the  nucleus,  and  a  small 
amount  of  cytoplasm  which  is  arranged  in  the  form  of 
irregular  pointed  projections.  They  are  very  sensitive 


FIG.  51. — Migration 
of  leukocytes. 


THE  BLOOD  123 

and  rapidly  disintegrate  when  removed  from  their  normal 
medium,  the  plasma.  For  this  reason,  they  are  rarely  ob- 
served in  blood  collected  in  glass  receptacles,  although  they 
may  be  preserved  for  some  time  in  agar  jelly  or  in  solutions 
of  magnesium  sulphate.  The  blood  of  the  frog,  fishes,  and 
birds  does  not  contain  these  elements. 

It  has  been  shown  that  the  platelets  play  an  important 
part  in  the  coagulation  of  the  blood.  When  disintegrating 
they  liberate  thrombokinase,  an  agent  which,  as  will  be 
shown  later,  is  primarily  responsible  for  this  change.  Those 
animals  which  are  not  in  possession  of  blood  platelets,  con- 
tain a  certain  coagulating  agent  in  their  tissues.  Conse- 
quently, their  blood  must  clot  as  readily  as  that  of  the 
animals  possessing  these  bodies,  because  in  its  escape  from 
the  vessel  it  must  flow  across  the  incised  tissues  and  be 
impregnated  with  this  substance.  Contrariwise,  if  the 
blood  of  a  frog  or  other  platelet-free  animal  is  collected  in  a 
receptacle  without  having  previously  come  in  contact  with 
the  tissues,  it  will  not  clot,  because  no  coagulating  substance 
has  been  added  to  it.  Inasmuch  as  Man  possesses  not  only 
blood  platelets  but  also  a  certain  amount  of  coagulating  sub- 
tance  within  his  tissues,  he  is  especially  well  protected. 

The  Coagulation  of  the  Blood. — Possibly  the  most  strik- 
ing characteristic  of  the  blood  is  its  power  of  changing  its 
fluid  consistency  into  one  of  semi-solidity.  Under  ordinary 
circumstances  it  remains  perfectly  fluid  while  traversing  the 
vascular  channels,  but  assumes  a  gelatinous  state  very  shortly 
after  it  has  been  brought  in  contact  with  a  foreign  body. 
The  platelets  are  then  destroyed  and  liberate  thrombokinase. 
This  agent  in  turn  incites  those  peculiar  changes  which 
gradually  cause  the  blood  to  lose  its  fluidity.  Now,  since 
the  blood  may  be  brought  in  contact  with  a  foreign  object 
while  still  in  the  bloodvessels  as  well  as  after  it  has  been  shed, 
it  is  possible  to  recognize  two  forms  of  coagulation,  namely, 
an  intravascular  one  and  an  extravascular  one. 

When  blood  is  allowed  to  flow  into  a  beaker,  it  is  at  first 
perfectly  fluid,  but  stiffens  perceptibly  after  the  expiration 
of  about  four  or  five  minutes.  The  time  intervening  be- 
tween the  moment  of  its  withdrawal  and  the  moment  when  it 


124        THE   CIRCULATION   OF   THE  BLOOD   AND   LYMPH 

first  assumes  a  gelatinous  consistency,  is  known  as  the 
coagulation-time.  While  the  length  of  this  period  varies 
somewhat  in  different  animals  and  is  subject  to  certain  con- 
ditions, such  as  the  temperature  and  carbon  dioxid  content  of 
the  blood  and  the  size  and  smoothness  of  the  vessel  into 
which  the  blood  is  drawn,  its  average  value  may  be  said  to 
be  5  minutes. 

Brief  reference  should  also  be  made  at  this  time  to  the 
fact  that  the  blood  of  some  persons  does  not  clot  until  after 
the  expiration  of  a  much  longer  period  than  the  one  just 
given.  Whenever  the  coagulation-time  is  unduly  prolonged, 
so  that  the  person  would  really  be  in  danger  of  suffering  an 
excessive  loss  of  blood  even  after  a  slight  injury,  he  is  said  to 
be  a  bleeder  or  hemophilic.  The  cause  of  this  condition  of 
hemophilia  is  not  known.  Peculiarly  enough,  it  affects 
only  the  males  of  a  particular  parent,  destroying  them  as 
a  rule  before  they  have  reached  middle  age.  The  females 
are  exempt,  but  may  propagate  this  characteristic  to  their 
children.  The  males  of  the  second  generation  are  subject 
to  the  same  difficulties,  while  the  females  survive.  In 
many  instances  these  severe  hemorrhages  take  place  without 
any  apparent  cause  and  are  repeated  at  intervals  until  life 
has  been  terminated. 

The  clotting  of  the  blood  is  due  to  the  production  of  fibrin, 
a  complex  substance  which  appears  in  the  form  of  delicate 
threads  permeating  the  blood  in  all  directions.  These 
filaments  arise  in  the  different  colonies  of  disintegrated 
blood  platelets  which  have  been  deposited  upon  the  sides  of 
the  receptacle.  In  their  progress  through  the  blood  they 
envelop  the  red  and  white  corpuscles,  finally  carrying  them 
by  gravity  to  the  bottom  of  the  receptacle.  This  mass  of 
fibrin  and  entrapped  corpuscular  elements  form  the  clot  or 
coagulum.  Above  it  lies  the  serum,  i.e.,  the  blood  plasma 
devoid  of  its  formed  elements  and  other  substances  used  up 
in  the  process  of  coagulation. 

Fibrin  is  derived  from  fibrinogen,  a  normal  constituent  of 
the  blood.  This  conversion  of  the  inactive  fibrinogen  into 
the  active  fibrin  is  brought  about  by  a  complex  ferment-like 
substance,  known  as  thrombin.  The  latter  in  turn  is  derived 


THE  BLOOD  •        125 

from  thrombogen,  a  normal  but  inactive  constituent  of  the 
blood.  The  activation  of  the  latter  is  accomplished  by 
thrombokinase  in  the  presence  of  calcium.  As  is  indicated 
in  the  accompanying  table,  the  destruction  of  the  platelets 
or  thrombocytes  liberates  thrombokinase  which  in  turn 
changes  the  thrombogen  into  thrombin,  but  only  in  the 
presence  of  calcium.  The  thrombin  then  converts  fibrinogen 
into  fibrin. 

Plasma  Blood  Corpuscles 


f~  ~~(  Platelets 

Fibrinogen  |  Calcium 


Thrombogen 

Thrombokinase 
*  Thrombin          *  ' 


>   Fibrin 

In  accordance  with  the  preceding  discussion,  it  may  be 
concluded  that  the  circulating  blood  remains  fluid  because 
it  is  everywhere  in  contact  with  the  normal  intima  of  the 
bloodvessels.  Obviously,  the  normal  lining  of  the  vessels  is 
not  destructive  to  its  formed  elements,  particularly  not  the 
platelets.  It  may  then  be  reasoned  that  any  injury  to  the 
vascular  channels,  rendering  a  segment  of  a  vessel  abnormal, 
must  lead  to  the  coagulation  of  the  blood,  because  the  part 
of  a  vessel  rendered  abnormal  by  a  stroke  or  chemical  sub- 
stance, must  act  as  a  foreign  body  and  cause  a  certain 
number  of  platelets  to  collect  upon  it.  Their  disintegration 
then  liberates  thrombokinase  which  finally  causes  the  forma- 
tion of  the  fibrin  threads.  The  latter  entrap  increasing 
numbers  of  red  cells  and  leukocytes  until  a  complete  coagu- 
lum  has  been  produced.  A  stationary  intra vascular  clot  is 


126        THE   CIRCULATION   OF   THE   BLOOD    AND   LYMPH 

known  as  a  thrombus.  The  blood  playing  constantly  against 
a  thrombus  may  cause  a  piece  of  it  to  become  separated 
from  the  main  mass  and  to  be  forced  onward  into  more  distant 
circulatory  channels.  This  floating  thrombus  or  embolus  may 
finally  become  lodged  in  one  of  the  smaller  tubules,  causing 
an  anaemia  and  functional  uselessness  of  the  cells  normally 
supplied  by  this  bloodvessel. 

The  Function  of  the  Blood  and  Lymph. — The  foregoing  ac- 
count must  have  shown  that  the  blood  and  lymph  possess 
practically  identical  functions,  because  they  serve  primarily 
the  purpose  of  common  carriers.  As  such  they:  (a)  convey 
nutritive  particles  to  the  tissues  and  take  the  waste  products 
away  from  them,  (6)  supply  the  tissues  with  oxygen  and  relieve 
them  of  their  carbon  dioxid,  (c)  serve  as  an  osmotic  medium 
in  which  these  different  interchanges  are  accomplished,  (d) 
play  an  important  part  in  the  regulation  of  the  body-tem- 
perature, (e)  contain  certain  elements  which  protect  the  body 
against  toxic  material,  and  (/)  distribute  the  products  of  the 
ductless  glands  to  the  different  tissues. 


CHAPTER  XII 


THE  GENERAL  ARRANGEMENT  OF  THE 
CIRCULATORY  SYSTEM 

Basic  Principles  of  the  Circulation. — Stress  has  previously 
been  placed  upon  the  fact  that  blood  is  a  tissue,  consisting 
of  certain  cellular  elements,  the  corpuscles,  and  a  certain 
quantity  of  intercellular  material, 
the  plasma.  However,  in  order 
that  this  tissue  may  be  able  to 
fulfill  the  function  of  a  common 
carrier,  it  cannot  remain  station- 
ary but  must  move  rapidly  from 
place  to  place  and  eventually 
return  to  its  starting  point. 
Obviously,  a  complete  circulation 
can  only  be  established  with  the 
aid  of  three  factors:  namely,  a 
system  of  recurrent  tubes,  a  cir- 
culatory fluid,  and  a  mechanism 
by  means  of  which  the  latter  is 
made  to  move.  Since  the  func- 
tion of  the  blood  has  been  con- 
sidered in  detail  in  the  preceding 
chapter,  we  are  now  in  a  better 
position  to  study  those  factors 
which  enable  it  to  complete  its 
circuitous  course  through  the 
different  parts  of  the  body. 

This  subject-matter  may  well 
be  introduced  by  a  brief  discus- 
sion of  the  fundamental  laws  pertaining  to  the  movement 
of  liquids  as  illustrated  by  Fig.  52.  The  heart  which  furnishes 
the  driving  force  for  the  blood,  first  appears  as  a  simple 

127 


FIG.  52. — Schema  of  simple 
circulatory  system.  /,  phase 
of  contraction;  II,  phase  of 
relaxation  of  heart;  A  and 
B,  valves  guarding*  cardiac 
orifices;  D,  arteries;  C,  capil- 
laries; E,  veins. 


128        THE    CIRCULATION    OF   THE   BLOOD    AND    LYMPH 

enlargement  of  a  certain  segment  of  the  vascular  system. 
The  walls  of  this  segment  are  considerably  thickened  by  the 
deposition  of  a  special  type  of  muscle  cells  possessing  auto- 
matic qualities.  This  statement  implies  that  this  muscle 
tissue  contracts  rhythmically  in  consequence  of  an  as  yet 
unknown  inherent  cause.  Now,  inasmuch  as  this  simple 
tubular  heart  is  modelled  after  an  ordinary  rubber  bulb,  its 
alternate  contractions  and  relaxations  may  be  reproduced 
by  compressing  such  a  bulb  at  regular  intervals  in  the  palm 
of  the  hand.  By  inference  it  may  be  concluded  that  the 
contraction  of  the  heart  must  lead  to  a  decrease  in  its  size, 
in  consequence  of  which  the  fluid  within  it  is  placed  under  a 
higher  pressure  than  that  prevailing  in  the  circular  tube 
without.  Accordingly,  the  fluid  within  the  heart  must  seek 
a  place  of  least  resistance  by  escaping  through  orifices  A  and  B. 
Later  on,  when  this  organ  relaxes,  the  size  of  its  chamber  is 
increased  and  hence,  the  pressure  within  it  must  at  this 
time  be  lower  than  that  in  the  tube.  The  fluid  then  flows 
back  into  the  heart  through  orifices  A  and  B. 

Clearly,  a  movement  of  this  kind  cannot  be  called  a  cir- 
culation, but  represents  merely  an  oscillatory  shifting  of  the 
fluid.  A  true  round-about  movement,  however,  can  easily 
be  imparted  to  it  if  orifices  A  and  B  are  equipped  with  valves, 
opening  in  the  direction  in  which  the  circulation  is  to  be 
established.  This  modification  having  been  made,  the  rise 
in  pressure  resulting  in  the  contracting  heart  must  open 
valve  A  and  close  valve  B.  The  fluid  then  leaves  this  cen- 
tral compartment  through  opening  A.  Contrariwise,  the 
relaxation  of  the  heart  must  close  valve  A  and  open  valve  B, 
thereby  permitting  the  fluid  to  return  through  orifice  B. 
In  this  way,  every  particle  of  the  fluid  is  made  to  traverse  the 
tube  in  its  entirety  and  always  in  a  direction  from  A  to  B. 

In  the  normal  circulatory  system,  the  channels  conveying, 
the  blood  away  from  the  heart  are  designated  as  arteries,  and 
those  returning  it  to  the  heart,  as  veins.  The  centralmost 
supply  tube  is  commonly  termed  the  aorta,  and  the  central- 
most  collecting  tube,  the  vena  cava.  It  is  also  to  be  re- 
membered that  the  arteries  divide  repeatedly  into  smaller 
tubes  which  are  known  as  arterioles,  and  these  in  turn  into 


ARRANGEMENT    OF    THE    CIRCULATORY    SYSTEM          129 

still  finer  tubules  which  are  called  capillaries.  On  the  other 
side  of  this  network  of  fine  tubules  lie  the  smaller  collecting 
channels  or  .venules.  The  latter  finally  unite  into  the 
veins,  and  these  in  turn  into  the  vena  cava. 

Accordingly,  the  entire  circulatory  system  may  be  divided 
into  three  parts:  namely,  into  arteries,  capillaries,  and  veins. 
In  accordance  with  their  size,  these  tubes  ma'y  again  be 
classified  as  arteries,  arterioles,  arterial  capillaries,  capillaries 
proper,  venous  capillaries,  venules,  and  veins.  It  has 
previously  been  noted  that  any  vessel  conveying  the  blood 
away  from  the  heart,  is  known  as  an  artery  and  any  vessel 
returning  the  blood  to  this  organ,  as  a  vein.  Hence,  the 
decisive  factor  in  this  terminology  is  the  direction  of  the 
bloodflow  and  not  the  character  of  the  blood.  Thus,  it  is  a 
well  known  fact  that  the  pulmonary  artery  conveys  venous 
blood  to  the  lungs,  while  the  pulmonary  vein  carries  the 
thoroughly  aerated  blood  from  these  organs  to  the  heart. 
If  it  is  desired,  however,  to  make  particular  reference  to  the 
fully  oxygenated  blood,  we  may  employ  the  adjective 
"arterial,"  whereas  the  adjective  "venous"  implies  that 
the  blood  has  lost  a  part  of  its  oxygen. 

The  Elementary  Heart. — In  order  to  be  able  to  obtain  a 
concise  idea  regarding  the  action  of  the  heart  of  the  mammals, 
it  seems  advantageous  to  refer  here  briefly  to  a  few  data 
derived  from  comparative  physiology.  As  has  been  stated 
above,  the  heart  first  appears  as  a  simple  bulbular  enlarge- 
ment of  a  particular  segment  of  the  general  circulatory 
system.  The  muscle  tissue  composing  its  wall  is  auto- 
matically active,  and  furthermore,  its  orifices  are  beset  with 
small  membranous  flaps  which  jointly  possess  the  action  of  a 
valve.  The  tubular  character  of  this  organ  is  most  con- 
spicuously betrayed  by  the  vermes.  In  these  animals,  the 
bloodvessels  traversing  the  ventral  and  dorsal  parts  of 
the  body,  are  connected  by  a  number  of  lateral  channels. 
The  walls  of  the  latter  as  well  as  those  of  the  neighboring  seg- 
ment of  the  dorsal  vessel,  are  automatically  contractile.  In 
this  species,  therefore,  the  heart  extends  over  a  large  segment 
of  the  vascular  system,  and  is  distinctly  tubular  in  its 
character. 


130        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

Very  similar  conditions  are  met  with  in  the  amphibians 
and  reptiles.  The  hearts  of  these  animals  also  exhibit  a 
tubular  outline,  but  embrace  three  successive  cavities  which 
are  connected  with  one  another  by  large  orifices.  It  is  noted 


fiNUS 


CAROTID 


FIG.  53. — The  circulation  in  the  amphibians  diagrammatically  outlined. 

first  of  all  that  the  veins  returning  the  blood  from  the  different 
parts  of  the  body,  unite  before  they  actually  enter  the  heart, 
and  form  here  a  vestibule-like  compartment  which  is  known 
as  the  venous  sinus.  This  chamber  opens  into  the  ante- 


ARRANGEMENT    OF    THE    CIRCULATORY    SYSTEM          131 

chamber  of  the  heart  or  auricle,  whence  we  pass  into  its 
main  chamber  or  ventricle.  The  orifices  connecting  these 
different  compartments,  are  guarded  by  valves  which  open 
only  in  the  direction  of  the  ventricle.  Furthermore,  the 
various  segments  of  this  heart  do  not  contract  simultaneously 
but  successively,  the  sinus  contracting  first  and  the  ventricle 
last.  Consequently,  the  orderly  flow  of  the  blood  through 
this  organ  depends  upon  two  factors :  namely,  the  successive 
contraction  of  the  different  cardiac  segments,  and  the 
proper  closure  and  opening  of  the  valves  guarding  the  afore- 
said orifices.  Regarding  the  second  factor,  it  should  be 
stated  at  this  time  that  the  contraction  of  the  auricle  closes 
the  valve  situated  between  this  chamber  and  the  sinus,  but 
opens  the  one  between  it  and  the  ventricle.  Likewise,  the 
contraction  of  the  ventricle  closes  the  auriculo-ventricular 
valve,  but  opens  the  one  situated  in  the  root  of  the  aorta. 

In  accordance  with  the  work  performed  by  the  different 
portions  of  this  simple  heart,  it  will  be  noted  that  their  walls 
possess  a  different  contractile  power,  as  is  indicated  by  their 
varying  thickness.  The  walls  of  the  sinus  and  auricles  are 
thin,  because  the  work  demanded  of  them  is  comparatively 
slight.  Their  function  is  to  pump  the  blood  into  the  adjoin- 
ing compartment,  but  this  transfer  is  accomplished  at  a  time 
when  the  receiving  chamber  is  in  the  state  of  rest.  The 
ventricle,  on  the  other  hand,  drives  the  blood  throughout  the 
systemic  vessels  and  must  overcome  the  high  resistance 
resident  in  these  tubules.  This  end  it  can  only  accomplish 
by  establishing  a  high  initial  pressure. 

After  its  ejection  from  the  heart,  the  blood  enters  a  small 
cavity,  the  walls  of  which  possess  a  certain  contractile  power 
in  order  to  augment  the  action  of  the  ventricle.  This  bulbu- 
lar  enlargement  of  the  roots  of  the  aortae  is  known  as  the 
arterial  bulb  or  bulbus  arteriosus.  Distally  to  it  the  two 
aortse  are  seen  to  convey  the  blood  to  all  parts  of  the  body, 
but  before  leaving  the  immediate  vicinity  of  the  heart,  these 
vessels  give  off  two  branches  which  lead  to  the  lungs.  By 
this  means  a  portion  of  their  contents  is  always  diverted  into 
the  capillaries  of  these  organs  to  be  oxygenated.  The  blood 
is  then  returned  to  the  left  auricle  by  special  vessels  which 


132        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

are  termed  the  pulmonary  veins.  Accordingly,  the  circu- 
latory system  of  the  frog  may  be  divided  into  a  greater  or 
systemic  circuit  and  a  lesser  or  pulmonary  circuit.  The 
former  supplies  the  different  tissues  of  the  body,  and  begins 
with  the  aortse.  It  terminates  finally  at  the  venous  sinus. 
The  latter  originates  with  the  pulmonary  branches  of  the 
aortse,  and  ends  at  the  left  auricle.  The  same  general  con- 
ditions prevail  in  the  mammals,  although  in  them  the  pul- 
monary artery  arises  directly  from  the  right  ventricle. 
Furthermore,  the  venous  sinus  does  not  appear  in  these 
animals  as  a  separate  cavity. 

It  need  scarcely  be  emphasized  that  the  perfectly  open 
condition  of  the  ventricular  cavity  permits  the  venous  blood 
of  the  right  auricle  to  intermingle  with  the  freshly  aerated 
blood  of  the  left  auricle.  It  is  to  be  noted,  however,  that  a 
thorough  mixture  between  these  two  types  of  blood  cannot 
take  place,  because  the  ventricle  contracts  immediately 
upon  the  completion  of  the  contraction  of  the  auricles. 
Moreover,  since  the  oxygenated  blood  of  the  left  auricle  is 
collected  in  that  portion  of  the  ventricular  cavity  which  lies 
nearest  the  aortse,  this  type  of  blood  must  leave  the  heart 
first  and  follow  the  path  of  least  resistance  into  the  distal 
arteries.  The  succeeding  gush  of  venous  blood,  on  the  other 
hand,  must  be  retained  very  largely  in  the  central  segments 
of  the  aortse,  whence  it  is  directed  into  the  pulmonary  arteries 
and  capillaries  of  the  lungs.  It  is  evident,  however,  that  a 
partial  mixing  of  the  oxygenated  and  venous  types  of  blood 
cannot  be  entirely  avoided  in  spite  of  this  peculiarity  in  the 
distribution  of  the  ventricular  blood.  Consequently,  the 
systemic  arteries  cannot  be  supplied  altogether  .  with 
thoroughly  aerated  blood. 

Very  similar  conditions  prevail  in  the  heart  of  the  turtle. 
It  is  evident,  however,  that  this  organ  is  more  solidly  built, 
than  that  of  the  frog  in  order  to  enable  it  to  produce  higher 
degrees  of  pressure.  Furthermore,  the  aortse  and  pulmon- 
ary artery  arise  directly  from  the  ventricle  as  separate  tubes. 
The  ventricle  itself  is  modified  to  show  an  at  least  partial 
separation  into  two  cavities.  This  separation  is  accom- 
plished by  two  membranous  flaps  which  are  brought  together 


ARRANGEMENT    OF    THE    CIRCULATORY    SYSTEM 


133 


at  their  edges  during  the  period  of  contraction  but  again 
move  away  from  one  another  during  the  subsequent  relaxa- 
tion of  this  portion  of  the  heart.  This  partition  serves  the 
purpose  of  keeping  the  blood  ejected  by  the  left  and  right 


FIG.  54. — The  circulation  in  reptilians  diagrammatically  outlined. 

auricles  apart,  so  that  the  former  may  be  directed  into  the 
aortse  and  the  latter  into  the  pulmonary  artery. 

The  Complex  Heart. — The  heart  of  the  adult  mammal  is 
divided  by  a  median  longitudinal  septum  into  a  right  and  a 


134         THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

left  half.  Each  half  is  in  turn  divided  by  a  transverse  septum 
into  an  antechamber  or  auricle  and  a  main  chamber  or 
ventricle.  Accordingly,  this  organ  consists  of  four  chambers : 
namely,  two  auricles  and  two  ventricles.  The  blood  re- 
turned by  way  of  the  systemic  veins  is  conveyed  into  the 
right  auricle,  while  the  blood  from  the  lungs  is  directed  into 
the  left  auricle.  The  two  arterial  channels  leaving  the  heart 
arise  from  its  ventricular  portion,  i.e.,  the  aorta  from  the  left 


FIG.  55. — Diagram  to  show  the  course  of  the  blood  through  the  higher 
heart.  PC,  post  cavse;  RA,  right  auricle;  -LA,  left  auricle;  RV, 
right  ventricle;  LV,  left  ventricle;  PA,  pulmonary  artery ;PV,  pulmon- 
ary vein;  A,  aorta. 

ventricle  and  the  pulmonary  artery  from  the  right  ventricle. 
Thus,  it  may  be  said  that  the  mammalian  heart  is  in  posses- 
sion of  two  venous  and  two  arterial  orifices,  although  the 
caval  and  pulmonary  openings  are  really  formed  by  two. 
tubes;  in  fact,  in  some  animals  four  pulmonary  veins  are 
present. 

Every  globule  of  blood  arriving  in  the  right  auricle  passes 
from  here  into  the  right  ventricle  and  thence  into  the  pul- 
monary artery.  After  it  has  traversed  the  capillaries  of  the 
lungs  it  is  returned  to  the  heart  by  way  of  one  of  the  pulmon- 


ARRANGEMENT    OF   THE    CIRCULATORY    SYSTEM         135 

ary  veins.  It  reenters  this  organ  at  the  left  auricle,  and 
subsequently  traverses  the  left  ventricle  in  order  to  reach 
the  aorta.  Distally  to  this  channel,  it  follows  the  general 
course  of  the  circulatory  system  and  finally  regains  the  heart 
by  way  of  one  of  the  vena  cava. 

Contrary  to  the  conditions  prevailing  in  the  lower  animals, 
the  adult  mammalian  heart  is  constructed  in  such  a  way  that 
an  intermingling  between  the  arterial  and  venous  blood  can- 
not take  place.  Hence,  its  left  side  contains  scarlet  red 
arterial  blood,  and  its  right  side  dark  venous  blood.  This 
separation,  however,  is  not  established  until  after  birth, 
because  before  this  time  a  communication  exists  between 
the  right  and  left  auricles  which  allows  the  blood  of  the  right 
cavity  to  escape  into  the  corresponding  compartment  on  the 
left  side.  But,  inasmuch  as  the  lungs  of  the  mammals  are 
absolutely  inactive  before  birth,  we  cannot  justly  speak  of  an 
intermingling  between  the  venous  and  arterial  blood.  It  is 
to  be  noted  that  the  circulatory  system  of  the  fcetus  contains 
fully  oxygenated  blood  only  in  the  umbilical  vein  which  con- 
veys the  blood  from  the  placenta  of  the  mother  towards  the 
fcetal  heart.  This  vessel  divides  into  two  branches,  which 
unite  with  the  inferior  vena  cava  and  the.  portal  vein.  Conse- 
quently, the  placental  blood  loses  its  arterial  character  at  some 
distance  from  the  heart  as  soon  as  it  is  mixed  with  the  venous 
blood  returned  from  the  posterior  parts  of  the  foetal  body. 
The  aforesaid  orifice  closes  as  a  rule  very  shortly  after  birth, 
but  may  also  persist  for  a  longer  period  of  time.  It  need 
scarcely  be  mentioned  that  those  infants  in  whom  this  orifice 
remains  widely  open,  permitting  an  intermingling  between 
the  freshly  aerated  and  non-aerated  blood,  cannot  survive, 
because  their  tissues  do  not  receive  the  required  amounts  of 
oxygen. 


CHAPTER  XIII 
THE  HEART  OF  THE  MAMMALS 

The  Cardiac  Cycle. — While  the  mammalian  heart  contains 
four  separate  chambers,  it  should  be  clearly  understood  that 
its  two  auricles  contract  together,  and  that  their  contraction 
precedes  that  of  the  ventricles  by  about  one-tenth  of  a  second. 
The  contraction  of  the  heart  is  known  as  systole,  and  its 
relaxation  as  diastole.  The  latter  period  is  followed  by  a 
distinct  phase  of  rest.  A  complete  beat  of  this  organ,  begin- 
ning with  the  contraction  of  the  auricles  and  ending  with 
the  pause  of  the  ventricles,  is  commonly  designated  as  a 
cardiac  cycle. 

Under  normal  circumstances,  the  heart  of  a  man  of  medium 
size  completes  about  70  cycles  in  one  minute,  while  that  of 
woman  beats  80  and  that  of  children  90  times  in  a  minute. 
Consequently,  each  cycle  must  consume  about  0.8  of  a  second, 
half  of  this  time  being  occupied  by  the  pause.  It  will  be 
seen,  therefore,  that  the  normal  heart  works  as  much  as  it 
rests.  When,  however,  its  frequency  is  greatly  increased, 
its  period  of  rest  is  materially  shortened,  so  that  an  organ 
beating  at  the  rate  of  about  140  times  in  a  minute,  would 
show  only  its  successive  systolic  and  diastolic  movements 
without  the  intervening  pauses.  The  heart  reacts  very 
promptly  to  changes  in  the  outside  temperature  and  tem- 
perature of  the  body.  Muscular  exercise  increases  its  rate 
considerably,  while  rest  and  sleep  decrease  it  to  something 
like  three-fourths  its  normal  rate. 

The  General  Arrangement  of  the  Valves  of  the  Heart. — 
The  preceding  discussion  pertaining  to  the  action  of  the 
simple  tubular  heart  has  brought  out  the  important  fact 
that  the  orderly  flow  of  the  blood  is  accomplished  by  two 
factors:  namely,  the  successive  contraction  of  the  different 
portions  of  this  organ,  and  the  proper  opening  and  closure 

136 


THE  HEART  OF  THE  MAMMALS 


137 


of  its  valves.  The  same  conditions  prevail  in  the  hearts  of 
the  mammals.  Each  cardiac  cycle  is  initiated  by  the  practi- 
cally simultaneous  contraction  of  the  auricles.  Immediately 
upon  the  completion  of  the  systole  of  these  chambers,  the 
ventricles  begin  their  systolic  movement.  While  this  wave 
of  activity  sweeps  over  this  organ  the  valves  guarding  its 
different  orifices  are  shifted  in  such  a  way  that  the  blood  must 
flow  from  the  auricles  into  the  ventricles,  and  thence  into 
the  large  arteries. 


Right  common 
carotid  artery. 

Subclavian 

arteries. 
Innominate 
^ artery. 

Arch  of  aorta. 
Right  lung. 

Superior  vena 
cava. 

Right  auricle. 


Larynx. 


—  Trachea. 
Subclavian 
arteries. 


Left  lung. 

Pulmonary 

artery. 


FIG.  56. — Relation    of    lungs    to    other    thoracic    organs.     (Ingals.) 


Having  become  acquainted  with  these  general  facts  per- 
taining to  the  action  of  the  heart,  we  are  now  in  a  better 
position  to  study  the  structure  and  arrangement  of  these 
valves  in  somewhat  greater  detail.  It  is  to  be  noted  first  of 
all  that  the  venous  entrances  to  the  heart  are  not  guarded 
by  valves,  although  the  size  of  these  openings  may  be  some- 
what diminished  by  the  contraction  of  those  muscle  fibers 
which  are  arranged  circularly  around  their  lumina.  These 
muscle  cells  act  somewhat  in  the  manner  of  the  stop  of  a 


138         THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

photographic  camera,  although  it  has  been  shown  that  they 
do  not  obliterate  these  passages  entirely.  In  fact,  such  a 
closure  is  quite  unnecessary  inasmuch  as  the  systole  of  the 
auricles  cannot  produce  a  pressure  high  enough  to  reverse 
the  venous  bloodstream. 

The  remaining  orifices  of  the  heart  are  all  guarded  by 
valves.     They  are  the  left  and  right  auriculo-ventricular 


FIG.  57. — Heart  of  the  cow,  with  left  auricle  and  ventricle  laid  open. 
a,  Root  of  the  aorta;  6,  spaces  in  the  wall  of  the  auricle;  c,  c,  orifices  of  the 
pulmonary  veins;  I,  I,  pulmonary  veins;  p,  p,  papillary  muscles;  q,  q, 
columnse  carnese.  A,  orifice  of  the  aorta;  K,  left  ventricle;  S,  septum; 
V,  left  auricle;  W,  lateral  wall  of  left  ventricle;  1,  1,  2,  leaflets  of  mitral 
valve.  (Miiller.) 

valves  and  the  aortic  and  pulmonary  valves.  Accordingly, 
the  valves  of  the  heart  may  be  arranged  in  two  sets :  namely, 
those  guarding  the  communications  between  the  auricles 
and  ventricles,  and  those  situated  in  the  beginning  portions 
of  the  aorta  and  pulmonary  artery.  The  former  are  desig- 
nated as  the  mitral  and  tricuspid,  and  the  latter  as  the  aortic 


THE    HEART    OF    THE    MAMMALS  139 

and  pulmonary  semilunar  valves.  The  mitral  valve  consists 
of  two  large  flaps,  the  basal  portions  of  which  are  attached 
to  the  margins  of  the  relatively  large  orifice  on  the  left  side, 
while  the  tricuspid  embraces  three  flaps  and  is  set  in  the 
somewhat  triangular  right  orifice.  Each  semilunar  valve 
embraces  three  cup-shaped  flaps,  their  convex  surfaces  being 
turned  toward  the  ventricles. 

auriculo-ventricular   set  (  m.itral  .(lef/>  , 

[  tricuspid  (right) 

Intracardiac   valves 


semilunar  set 

(  pulmonary 

Besides  these  intracardiac  valves,  the  vascular  system 
is  also  in  possession  of  a  large  number  of  valves  which  guard 
the  orifices  of  the  smaller  veins  at  their  points  of  union  with 
larger  ones.  These  venous  valves  usually  consist  of  two  cup- 
shaped  flaps  which  open  only  in  the  direction  of  the  heart, 
and  close  immediately  if  the  pressure  in  the  more  central 
vein  rises  above  that  prevailing  in  the  tributary  vessel. 

The  Action  of  the  Cardiac  Valves.  —  Inasmuch  as  the 
orifices  between  the  auricles  and  ventricles  are  large,  while 
those  leading  into  the  main  arteries  are  relatively  small,  it 
need  not  surprise  us  to  find  that  these  valves  present  certain 
structural  differences  in  accordance  with  their  location. 
In  the  first  place,  it  should  be  noted  that  the  several  seg- 
ments of  each  valve  are  formed  by  simple  duplications  of  the 
general  lining  membrane  of  the  heart,  known  as  the  endo- 
cardium. But  in  order  to  give  a  greater  stability  to  these 
flaps,  a  certain  amount  of  fibrous  tissue  and  a  few  elastic 
fibers  have  been  added  to  their  framework.  As  has  just 
been  stated,  the  bicuspid  or  mitral  valve  consists  of  two 
large  flaps,  whereas  the  tricuspid  valve  embraces  three 
segments.  This  arrangement  is  in  keeping  with  the  general 
outline  of  these  orifices,  that  on  the  left  side  being  oval  and 
that  on  the  right  side  somewhat  triangular. 

While  these  valve  flaps  are  surprisingly  thin,  they  are 
nevertheless  very  strong,  and  cannot  easily  be  torn  with  the 
fingers.  Their  basal  zones  are  fastened  to  the  walls  of  the 
orifice,  while  their  tips  are  somewhat  pointed  and  project 


140        THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

free  into  the  lumen  of  the  cavity.  When  a  valve  is  com- 
pletely closed,  its  different  flaps  assume  a  position  trans- 
versely across  the  orifice  and  are  firmly  united  with  one 
another  at  their  margins.  Their  thorough  approximation  is 
accomplished  with  the  aid  of  threads  of  tendinous  tissue, 
known  as  the  chordae  tendinece.  When  the  cavity  of  either 
ventricle  is  opened,  it  will  be  noted  that  its  surface  is  very 
uneven,  and  presents  many  prominent  longitudinal  strands 


FIG.  58. — Schema  to  show  fan-like  distribution  of  chordae  tendineae 
(CT)  from  a  single  papillary  muscle  (P),  situated  underneath  (V),  two 
adjoining  valve  flaps. 

of  muscle  tissue  which  are  designated  as  columns  of  flesh  or 
columnce  carnece.  Several  of  these  columns  extend  only  about 
half-way  up  the  ventricular  wall,  and  terminate  at  this 
level  in  the  form  of  rounded  prominences.  These  muscular 
projections  are  termed  papillary  muscles.  A  fact  of  greatest 
interest  to  us  at  this  time  is  that  these  structures  serve  as 
points  of  attachments  for  the  tendinous  cords  which  extend 
from  here  in  an  upward  direction  to  establish  connections 
with  the  under  surfaces  of  the  different  valve  flaps.  In  their 
course  through  the  ventricular  cavity  these  cords  divide  and 
subdivide  repeatedly  into  a  delicate  fan-shaped  network. 
The  chordae  tendinese  possess  a  definite  length,  which 
equals  the  distance  between  the  papillary  muscles  and  the 


THE   HEART   OF   THE    MAMMALS  141 

different  valve  flaps  when  in  the  position  of  closure.  It  is 
needless  to  state  that  their  function  is  to  hold  the  latter 
firmly  in  place,  so  that  they  cannot  be  diverted  into  the 
auricles.  Consequently,  they  serve  the  same  purpose  as 
the  guy-rope  of  the  sail-beam  which  allows  the  sail  to  be 
deflected  by  the  wind,  holding  it  firmly  in  place  as  soon  as  it 
has  reached  its  proper  position. 

Another  very  interesting  structure  found  in  the  right 
ventricle  of  certain  animals,  is  the  so-called  moderator  band. 
This  structure  presents  itself  as  a  rather  thick  strand  of 
tissue  which  is  stretched  transversely  across  the  cavity  from 
the  interventricular  septum  to  the  outer  wall.  Obviously, 


FIG.  59. — Longitudinal  section  through  the  root  of  the  aorta  showing  cup- 
like  shape  of  semilunar  valve  flaps. 

the  purpose  of  this  cord  is  to  hold  the  relatively  thin  wall 
of  the  right  ventricle  in  place,  so  that  it  cannot  be  unduly 
distended.  Inasmuch  as  the  wall  of  the  left  cavity  is  very 
thick,  it  is  not  in  need  of  a  supporting  structure  of  this  kind. 
The  arterial  orifices  of  the  heart  are  comparatively  small 
in  size.  Their  walls  are  beset  with  three  cup-shaped  flaps, 
the  concave  surfaces  of  which  are  turned  outward.  No 
special  mechanism  in  the  form  of  tendinous  cords  is  required 
to  hold  them  in  place,  because  their  margins  support  one 
another  when  the  valve  is  closed.  During  the  phase  of 
contraction  of  the  ventricles,  these  flaps  are  turned  outward, 
thereby  allowing  the  blood  to  escape  into  the  arteries.. 
Contrariwise,  the  relaxation  of  the  ventricles  and  consequent 
back  pressure  of  the  arterial  blood  cause  their  margins  to 
snap  together  immediately.  Being  in  this  way  prevented 


142         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

from  returning  into  the  ventricles,  the  blood  must  follow 
the  course  of  least  resistance  through  the  capillaries  and 
veins. 

In  order  to  insure  an  orderly  direction  of  the  flow  through 
the  heart,  the  auriculo-ventricular  and  semilunar  sets  of 
valves  must  move  in  opposite  directions  to  one  another,  but, 
naturally,  their  movements  must  remain  the  same  on  the 
two  sides.  It  will  be  seen  that  the  systole  of  the  auricles 
deviates  the  auriculo-ventricular  valve-flaps  in  a  downward 


FIG.  60. — Diagram  to  show  position  of  semilunar  valve  flaps  on  closure. 
/,  longitudinal  section;  77,  transverse  section;  V,  ventricle;  A,  aorta; 
FV,  fossa  of  Valsalva;  C,  corpora  Arantii. 

direction,  i.e.,  toward,  but  not  against,  the  inner  surfaces 
of  the  walls  of  the  ventricles.  During  this  period  the  con- 
tents of  the  auricles  are  quickly  forced  into  the  ventricular 
cavities.  Scarcely  has  this  act  been  completed  when  the 
ventricles  begin  their  systolic  phase.  As  the  pressure  within 
these  chambers  rises  the  auriculo-ventricular  valve  flaps 
are  shifted  into  the  position  of  closure,  thereby  preventing 
the  blood  from  flowing  into  the  auricles.  The  pressure  in 
the  ventricles  now  rises  very  rapidly  until  it  finally  exceeds 
that  prevailing  in  the  arteries.  At  this  very  moment,  the 
flaps  composing  the  semilunar  valves  are  turned  outward, 
thereby  permitting  the  blood  to  escape  into  the  arteries. 
Immediately  upon  the  completion  of  the  systole  of  the 
ventricles,  the  semilunar  valves  are  closed,  because  at  this 
time  the  arterial  pressure  exceeds  that  prevailing  in  the 
ventricles. 


THE    HEART    OF    THE    MAMMALS  143 

As  soon  as  the  auricles  have  completed  their  contraction, 
the  high  pressure  existing  in  the  veins  causes  the  blood  to 
enter  their  cavities.  Finally,  when  a  certain  passive  disten- 
tion  of  their  walls  has  been  attained,  the  auriculo-ventricular 
valves  open  and  allow  a  certain  portion  of  the  auricular 
blood  to  escape  into  the  ventricles.  Consequently,  the 
auricular  and  ventricular  cavities  are  already  well  filled 


FIG.  61. — Diagram  illustrating  the  position  of  the  cardiac  valves  during 
(A)  auricular  systole  and  (V)  ventricular  systole.  Only  one-half  of  the 
heart  is  represented. 

before  the  next  systole  of  the  auricles  sets  in.  The  contrac- 
tion of  the  auricles,  however,  serves  to  fill  the  ventricles 
to  their  utmost  capacity. 

The  Manner  of  Excitation  of  the  Different  Segments  of 
the  Heart. — When  the  action  of  the  simple  tubular  heart  is 
studied  in  detail,  it  will  be  noted  that  its  contraction  begins  at 
the  entrance  of  the  large  veins  and  spreads  from  here  in 
the  form  of  a  wave  to  the  adjoining  auricles  and  ventricle. 
Consequently,  this  Organ  may  be  said  to  contract  in  a 
manner  similar  to  that  of  the  intestine,  i.e.,  peristaltically. 
It  has  been  stated  above  that  the  mammalian  heart  does 
not  possess  a  distinct  sinus  portion,  but  a  closer  observation 
will  show  immediately  that  its  contractions  also  begin  at  the 
point  of  entrance  of  the  venae  cavse.  This  region  embraces 
a  small  complex  of  tissue  which  is  peculiarly  receptive  to 


144         THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

certain  forms  of  stimuli  and  transfers  the  resultant  waves  of 
excitation  to  other  areas  of  the  auricles.  Because  of  its 
power  of  initiating  the  heart  beat,  it  is  usually  designated  as 
the  pace-maker  of  this  organ. 

The  wave  of  excitation  of  the  auricles  is  transferred  to 
the  ventricles  over  a  bridge  of  specialized  tissue  which  cannot 
be  classified  as  nervous  tissue,  nor  as  muscle  tissue.  It  is 
known  as  the  bundle  of  His.  The  upper  extremity  of  this 
connecting  band  of  tissue  is  indicated  by  a  nodular  enlarge- 


FIG.  62. — The  conduction  system  of  the  heart.  SC,  superior  vena 
cava;  1C,  inferior  vena  cava;  RA,  right  auricle ;L A,  left  auricle;  RV,  right 
ventricle;  LV,  left  ventricle;  PA,  pulmonary  artery;  A,  aorta;  PV,  pul- 
monary veins;  PM,  pace-maker;  IN,  interauricular  node;  B,  bundle  of 
His;  P,  fibers  of  Purkinje. 

ment  situated  in  the  wall  between  the  auricles.  It  is  desig- 
nated as  the  interauricular  node.  The  main  bundle  divides 
below  into  two  branches  which  finally  connect  with  the 
musculature  of  the  ventricles  by  an  intricate  network  of. 
delicate  fibers,  commonly  called  the  fibers  of  Purkinje.  It 
appears  therefore,  that  the  different  portions  of  the  heart 
are  activated  consecutively  in  consequence  of  an  excitation 
developed  in  the  pace-maker. 

The  substance  of  the  heart  is  made  up  of  three  layers: 
namely,  an  internal  lining  membrane  or  endocardium,  a 


THE    HEART   OF   THE   MAMMALS  145 

median  coat  of  muscle,  tissue  or  myocardium,  and  an  outer 
covering  or  epicardium.  The  outer  limiting  membrane  is 
reflected  from  the  large  vessels  at  the  base  of  this  organ  to 
form  a  pouch,  the  external  wall  of  which  is  termed  the  peri- 
cardium. By  this  reflection  a  narrow  space  is  cut  off  from 
the  general  cavity  of  the  thorax  which  is  called  the  peri- 
cardial  sac.  A  small  quantity  of  a  lymph-like  fluid  is  con- 
tained therein,  which  lubricates  the  inner  surfaces  of  this 
sac,  so  that  the  heart  may  change  its  shape  and  position  with 
the  least  possible  friction  and  resistance. 

The  Sounds  of  the  Heart. — The  contraction  of  every 
muscle  is  attended  by  certain  slight  movements  of  its  con- 
stituent fibers.  In  consequence  of  this  displacement,  an 
intercellular  friction  results  which  gives  rise  to  a  noise. 
When  the  mass  of  contracting  muscle  tissue  is  large,  this 
noise  may  be  perceived  with  the  naked  ear,  while  under  less 
favorable  conditions  it  becomes  necessary  to  employ  deli- 
cate amplifying  devices.  The  heart  does  not  form  an  excep- 
tion to  this  rule,  because  the  contraction  of  its  musculature 
produces  a  very  characteristic  sound  which  is  clearly  audible 
and  may  be  registered  by  means  of  micro-phonographic 
appliances.  Even  small  excised  segments  of  the  ventricles 
yield  a  sound  when  they  contract,  but  its  audibility  is 
correspondingly  decreased. 

It  is  a  well  known  fact  that  the  ear  when  applied  to  the 
region  of  the  apex  of  a  person's  heart,  perceives  two  distinct 
sounds  during  each  cardiac  cycle.  These  sounds  may  be 
transferred  in  almost  their  complete  intensity  by  means  of 
an  instrument  which  is  known  as  a  stethophone.  Their  practi- 
cal importance  is  based  upon  the  fact  that  they  betray  to  us 
not  only  the  frequency  of  the  heart,  but  also  the  manner  of 
action  of  its  different  valves.  It  has  been  thoroughly  estab- 
lished that  any  impairment  in  the  opening  or  closure  of  their 
several  valve-flaps  gives  rise  to  very  characteristic  changes 
in  the  quality  of  the  normal  sounds.  These  murmurs  must 
be  studied  by  the  physician  with  the  greatest  care  in  order 
to  be  able  to  determine  the  character  and  location  of  the 
valvular  lesion. 

The  first  sound  is  heard  during  the  period  of  contraction 
10 


146        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

of  the  ventricles,  and  possesses  a  rather  low  pitch  and  dull 
quality.  The  second  sound  is  noted  at  the  very  beginning  of 
the  diastolic  phase  and  presents  a  sharp,  snappy  character. 
Both  sounds  may  be  represented  by  the  syllables  "lubb- 
dupp"  or  "ta-ta."  More  recently,  a  third  sound  has  been 
discovered,  which  is  quite  inaudible  in  most  persons,  but 
may  be  perceived  in  all  individuals  by  means  of  sensitive 
amplifying  instruments. 

The  cause  of  the  first  sound  lies  in  the  contraction  of  the 
ventricular  musculature,  although  it  cannot  be  denied  that 
its  quality  is  slightly  modified  by  the  closure  of  the  mitral 
and  tricuspid  valves.  The  cause  of  the  second  sound  is 
wholly  valvular,  because  no  muscular  activity  occurs  during 
diastole.  The  valvular  change  effected  at  this  time  is  the 
closure  of  the  semilunar  valves.  Hence,  it  may  be  con- 
cluded that  the  second  heart  sound  is  occasioned  by  the 
closure  of  the  aortic  and  pulmonary  valves. 

The  Apex  Beat  or  Cardiac  Impulse. — When  the  surface  of 
the  chest  is  inspected,  it  will  be  observed  that  the  region  of  the 
apex  of  the  heart  bulges  outward  with  every  contraction  of 
the  ventricles.  The  area  so  affected  measures  about  2  cm. 
in  diameter  and  is  situated  in  men  in  the  fifth  intercostal 
space  slightly  below  and  to  the  right  of  the  left  nipple.  In 
woman  this  protusion  is  more  frequently  observed  in  the 
fourth  intercostal  space.  Its  cause  is  to  be  sought  in  the 
impact  of  the  systolic  ventricles  against  the  wall  of  the  chest. 
To  the  physician  the  location  and  character  of  this  impact 
betray  changes  in  the  size  of  the  heart  or  of  its  several  com- 
partments as  well  as  alterations  in  the  strength  of  its  beat. 

Valvular  Lesions  of  the  Heart. — It  need  scarcely  be  em- 
phasized that  the  improper  closure  of  any  one  of  the  valves 
of  the  heart  must  lead  to  an  impairment  in  the  flow  of  the 
blood  through  its  chambers  and  hence,  also  to  disturbances  in 
the  peripheral  circulation.  The  cardiac  valves  may  become 
insufficient  in  their  action  for  two  reasons:  Thus,  the  flaps 
may  lose  their  soft,  yielding  texture  in  consequence  of  in- 
filtrations, and  cease  to  move  freely  upon  their  basal  por- 
tions. Since  their  tips  can  then  no  longer  be  forced  widely 
apart,  the  orifice  guarded  by  them  must  be  greatly  diminished 


THE    HEART    OF    THE    MAMMALS 


147 


in  size.  This  condition  which  is  known  as  stenosis,  may  also 
be  occasioned  by  the  partial  growing  together  of  the  margins 
of  the  adjoining  valve-flaps.  Although  usually  inherited, 
this  defect  may  also  be  acquired  in  the  course  of  inflamma- 
tions of  the  lining  of  the  heart,  designated  as  endocarditis. 


XII 


FIG.  63. — Showing  location  of  apex  beat.  The  positions  of  the  aortic 
semilunar  (.+)  and  mitral  (A)  valves  are  indicated  in  red  and  those  of  the 
pulmonary  semilunar  ( +)  and  tricuspid  (A)  in  blue. 


Secondly,  the  flaps  cannot  be  made  to  close  perfectly,  because 
their  margins  are  not  fully  approximated.  A  small  opening 
is  then  left  between  them  through  which  the  blood  escapes 
backward  into  the  cavity  from  which  it  has  just  been  ejected. 
This  condition  is  known  as  regurgitation.  Almost  any 
valve  may  be  the  seat  of  a  lesion  of  this  kind,  although  the 
mitral  is  more  frequently  affected  than  the  others.  In 


148        THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

general,  therefore,  it  may  be  said  that  a  stenotic  valve  is 
altogether  too  unyielding,  whereas  a  regurgitating  valve  is 
really  too  flexible.  Thus,  one  of  its  chordae  may  have  been 
subjected  to  a  strain  with  the  result  that  the  corresponding 
flap  now  assumes  a  position  beyond  that  occupied  by  it 
during  the  normal  closure  of  the  orifice.  In  many  instances, 
however,  the  flaps  of  a  regurgitating  valve  show  nodular 
growths  upon  their  margins  which  prevent  their  perfect 
approximation. 

When  the  musculature  of  any  portion  of  the  heart  is 
forced  to  drive  the  blood  through  too  narrow  a  valvular 
orifice,  it  gradually  increases  in  size.  This  condition  is 
known  as  hypertrophy.  Obviously,  this  increase  in  its  vol- 
ume and  pumping  force  enables  it  to  sustain  the  circulation 
in  spite  of  the  aforesaid  resistance  to  the  flow.  The  same 
change  follows  regurgitating  lesions,  because  if  a  portion  of 
the  blood  is  always  returned  into  the  cavity  from  which  it 
has  just  been  ejected,  the  musculature  of  this  segment  must 
do  more  work  in-  order  to  be  able  to  deliver  a  sufficient 
quantity  of  blood  to  the  arteries.  Very  similar  conditions 
prevail  in  a  pump  that  possesses  a  leaky  valve.  We  well 
know  that  we  must  pump  much  more  forcefully  and  oftener 
at  this  time  in  order  to  collect  the  required  amount  of  water. 

A  heart  which  overcomes  the  effects  of  these  valvular 
lesions  by  hypertrophy  and  other  means,  such  as  an  increase 
in  its  frequency  of  contraction,  is  said  to  be  compensating 
and  may  last  for  many  years,  although  constantly  acting 
close  to  its  functional  limit.  When  a  heart  fails  to  hyper- 
trophy or  has  reached  its  limit  of  endurance,  owing  to  con- 
tinued strain,  it  often  dilates  very  abruptly,  thereby  putting 
an  end  to  the  circulation  and  life.  A  dilated  organ  is  large 
in  size,  but  its  walls  are  relatively  thin.  This  fact  indicates 
that  its  muscular  elements  have  been  excessively  distended 
and  are  no  longer  able  to  act  even  against  the  ordinary 
resistances  of  the  vascular  channels. 

The  Electrical  Energy  Liberated  by  the  Beating  Heart. — 
Attention  has  previously  been  called  to  the  fact  that  living 
matter  liberates  three  principal  forms  of  energy:  name- 
ly, mechanical  energy,  heat,  and  electricity.  The  heart 


THE    HEART    OF    THE    MAMMALS 


149 


furnishes  mechanical  energy  in  the  form  of  the  pressure 
required  to  drive  the  blood  through  the  vascular  system. 
The  heat  evolved  by  it  is  measurable,  but  does  not  exceed 
that  liberated  by  skeletal  muscle.  Lastly,  its  electrical 
energy  may  be  detected  by  connecting  the  exposed  heart 
of  an  animal  with  the  poles  of  an  electrical  indicator,  such 
as  a  string-galvanometer. 


FIG.  64. — Einthoven's  string  galvanometer,  as  modified  by  H.  B. 
Williams.  The  front-cover  has  been  removed  to  show  the  position  of  the 
string  between  the  poles  of  the  magnet.  The  connecting  posts  lie  behind 
the  hood  containing  the  string. 

In  very  recent  years,  a  method  has  been  devised  by  means 
of  which  it  is  possible  to  observe  and  measure  the  electrical 
variations  produced  in  the  normal  human  body  in  conse- 
quence of  the  activity  of  the  heart.  The  hands  of  the  subject 
are  placed  in  receptacles  containing  a  solution  of  zinc  sul- 
phate. Each  hand  firmly  grasps  a  platinum  electrode  which 


150        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

is  connected  with  the  string-galvanometer  by  means  of  a 
copper  wire.  The  galvanometer  consists  of  a  powerful 
magnet  between  the  poles  of  which  is  suspended  a  fiber  of 
quartz  coated  with  silver.  When  the  electrical  differences 
produced  in  the  body  by  the  beat  of  the  heart  are  allowed  to 
act  upon  this  magnet,  the  indicator  is  deflected  in  a  very 
characteristic  manner.  These  deflections  may  be  observed 
by  means  of  a  microscope  and  may  be  projected  into  a  camera 
to  be  photographed.  The  different  appliances  required  to 
obtain  this  record  of  the  activity  of  the  heart  constitute  the 
electrocardiograph.  With  the  help  of  this  instrument  it  is 
possible  to  detect  any  irregularity  in  the  beat  of  the  heart 
and  to  obtain  a  fair  idea  regarding  the  cause  of  the  latter. 
The  Size,  Weight  and  Position  of  the  Human  Heart. — The 
adult  human  heart  is  lodged  in  the  mediastinal  space  be- 
tween the  lungs,  nearer  the  ventral  than  the  dorsal  wall 
of  the  chest.  It  is,  roughly  triangular  in  shape,  its  basal 
portion  being  directed  upward,  backward  and  to  the  right, 
while  its  tip  or  apex  is  turned  downward,  forward  and  to  the 
left.  Consequently,  the  longitudinal  axis  of  this  organ  is 
directed  obliquely  from  a  point  situated  about  1J^  inches  to 
the  right  of  the  mid-sternal  line  at  the  second  rib,  to  a  point 
about  3J^  inches  to  the  left  of  this  line  in  the  fifth  intercostal 
space.  In  the  adult  male,  the  heart  measures  125  mm.  in 
length,  87  mm.  in  breadth,  and  62  mm.  in  thickness.  Its 
weight  is  approximately  310  grams  and  its  volume  250  c.c. 
Its  size  may  be  said  to  equal  that  of  the  closed  fist  of  the 
person  to  whom  it  belongs.  The  heart  of  the  adult  female 
weighs  250  grams. 


CHAPTER  XIV 
THE  FLOW  OF  THE  BLOOD 

The  Circulatory  System. — The  greater  and  lesser  circuits 
of  the  vascular  system  arise  from  the  heart  as  single  tubes, 
their  repeated  division  and  subdivision  eventually  giving 
rise  to  a  large  number  of  arteries,  arterioles,  and  capillaries. 
For  this  reason,  the  arterial  system  has  been  likened  to  a 
tree,  the  trunk  of  which  corresponds  to  the  aorta,  while  its 
branches  may  be  said  to  represent  the  smaller  subdivisions 
of  this  supply  tube.  This  comparison  is  a  very  appropriate 
one,  although  it  does  not  represent  the  entire  system.  It 
leaves  out  of  account  the  collecting  channels  which  usually 
pursue  a  course  parallel  to  the  arteries.  Inasmuch  as  the 
student  is  frequently  at  a  loss  to  obtain  a  clear  picture  of 
the  vascular  systems  from  anatomical  diagrams  and  descrip- 
tions, it  seems  advisable  at  this  time  to  introduce  a  schema, 
such  as  is  represented  by  Fig.  65,  which  deals  with  the  many 
bloodvessels  as  if  they  were  single  tubes. 

The  greater  or  systemic  circuit  begins  with  the  aorta. 
This  vessel  sends  branches  towards  the  head  as  well  as 
towards  the  feet.  In  either  case,  the  blood  successively 
traverses  certain  arteries,  arterioles,  capillaries,  venules, 
and  veins,  and  finally  returns  to  the  right  auricle  and  ventri- 
cle. After  its  ejection  into  the  pulmonary  artery  it  passes 
through  the  different  vessels  of  the  lesser  or  pulmonary  cir- 
cuit, and  re-enters  the  heart  at  the  left  auricle.  As  has  been 
stated  above,  the  aorta  supplies  all  the  tissues  and  organs 
of  the  body,  excepting  the  lungs.  In  the  tissues  the  blood 
loses  a  portion  of  its  oxygen  and  acquires  carbon  dioxid. 
The  blood  retains  this  character  until  it  has  been  diverted 
into  the  capillaries  of  the  lungs,  where  it  discharges  its 
carbon  dioxid  and  acquires  oxygen. 

151 


152        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 


FIG.  65. — Schema  of  the  circulation.  A,  aorta;  Ar,  arteries;  Art, 
arterioles;  AC,  arterial  capillaries;  C,  capillaries;  VC,  venous  capillaries; 
Ven,  venules;  Ve,  veins;  VCS,  vena  cava  superior;  VCJ,  vena  cava  in- 
ferior; R A,  right  auricle;  RV,  right  ventricle;  LA,  left  auricle;  LV,  left 
ventricle;  1,  tricuspid  valve;  2,  mitral  valve;  3,  pulmonary  semil.  valve; 
4,  aortic  semil.  valve;  PA,  pulmonary  artery;  L,  lungs;  PV,  pulmonary 
veins;  PO,  portal  organs;  PV,  portal  vein;  HA,  hepatic  artery;  Li,  liver; 
HV,  hepatic  vein. 


THE  FLOW  OF  THE  BLOOD 


153 


It  should  also  be  remembered  that  the  systemic  or  greater 
circuit  embraces  two  smaller  divisions  which  exhibit  certain 
details  not  presented  by  the  others.  The  most  striking  de- 


FIG.  66. — Diagram  to  illustrate  the  changes  in  the  cross-section  of  the 
vascular    system.     A,  aorta;  Ar,  arteries;  C,   capillaries;   V,  veins;   VC, 


vena  cava. 


parture  from  the  usual  arrangement  is  made  by  the  blood- 
vessels supplying  the  portal  organs.  The  group  of  organs 
here  referred  to  includes  the  stomach,  intestine,  pancreas, 
spleen,  and  liver.  Each  of  these  is  supplied  with  arterial 


FIG.  67. — Schematic  representation  of  the  portal  circuit.  A,  abdomi- 
nal aorta;  C,  cceliac  axis  supplying  portal  organs;  HA,  hepatic  artery 
supplying  the  frame  work  of  the  liver.  PO,  portal  organs  (stomach, 
intestines,  spleen,  and  pancreas);  PV,  portal  vein;  L,  liver;  HV,  hepatic 
veins;  IV C,  inferior  vena  cava. 

blood  which  is  derived  from  the  coeliac  and  mesenteric 
branches  of  the  abdominal  division  of  the  aorta.  Their 
venous  drainage,  however,  is  not  collected  by  separate 
channels  but  by  a  single  one,  which  is  called  the  portal  vein. 


154         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

In  the  liver  this  vein  divides  into  many  smaller  branches 
which  in  turn  give  rise  to  an  intricate  network  of  capillaries. 
Centrally  to  this  organ,  the  hepatic  veins  convey  this  blood 
into  the  inferior  cava.  This  arrangement  allows  the  cells 
of  the  liver  to  make  first  use  of  the  nutritive  material  which 
has  traversed  the  lining  of  the  alimentary,  canal  before  it  is 
permitted  to  enter  the  general  circulation.  The  substances 
here  worked  over  are  the  simple  sugars  and  ammo-acids. 


FIG.  68. — The  heart.      (Stoney.) 

It  will  be  shown  later  that  the  sugar  is  stored  in  the  cells  of 
the  liver  in  the  form  of  glycogen,  while  the  amino-acids 
are  employed  in  the  production  of  urea,  an  important 
constituent  of  urine. 

The  bloodvessels  of  the  heart  form  an  exception  to  the 
general  arrangement  only  insofar  as  they  arise  from  the  root 
of  the  aorta  and  connect  directly  with  the  right  auricle. 
This  circuit  which  is  set  aside  for  the  nutrition  of  the  sub- 
stance of  this  organ,  is  known  as  the  coronary.  It  begins 
as  a  rule  with  two  arteries,  the  orifices  of  which  lie  behind  the 


THE    FLOW    OF    THE   BLOOD  155 

semilunar  flaps,  and  forms  a  superficial  as  well  as  a  deep  set  of 
smaller  vessels.  The  coronary  veins  eventually  unite  into  a 
single  channel  which  is  known  as  the  coronary  sinus.  It 
empties  directly  into  the  right  auricle. 

It  will  be  seen,  therefore,  that  the  contents  of  the  aorta 
are  diverted  into  a  very  large  number  of  vessels.  Possibly 
the  shortest  route  that  may  be  selected  by  the  blood  in  its 
return  journey  to  the  right  auricle,  is  the  coronary  circuit. 
A  much  longer  time  is  required  for  its  passage  through  the 
portal  organs,  kidneys,  and  extremities. 

The  Structure  of  the  Bloodvessels. — The  walls  of  the 
arteries  are  made  up  of  three  layers  of  tissue:  namely,  an 
inner  lining  of  epithelium  or  intima,  a  middle  coat  of  smooth 
muscle  cells  intermingled  with  connective  tissue,  and  an 
outer  layer  of  connective  tissue.  By  far  the  largest  number 
of  muscle  cells  are  arranged  circularly  around  the  lumen  of 
the  vessel.  They  are  particularly  numerous  in  the  arterioles, 
while  in  the  central  arteries  the  elastic  connective  tissue 
is  present  in  preponderating  amounts.  Accordingly,  it 
may  be  surmised  that  the  aorta  serves  the  purpose  of  an 
elastic  reservoir,  while  the  peripheral  vessels  possess  the 
power  of  influencing  the  bloodstream  in  an  active  manner 
by  lessening  the  size  of  the  channel. 

The  capillaries  possess  very  thin  walls  which  are  composed 
of  plate-like  endothelial  cells  cemented  together  at  their 
edges  by  intercellular  substance.  Each  cell  is  equipped  with 
a  small  oval  nucleus.  The  diameter  of  these  tubules  varies 
considerably,  some  of  them  being  so  small  that  the  red  cor- 
puscles cannot  enter  them  without  being  considerably 
elongated.  Others,  again,  permit  two  or  three  of  these 
elements  to  pass  side  by  side.  Consequently,  it  may  be  said 
that  their  average  diameter  is  something  like  15/z  or  Mooo 
of  an  inch.  Their  average  length  is  1  mm.  They  appear  as 
small-meshed  networks,  in  the  interspaces  of  which  lie  the 
individual  tissue  cells.  It  should  be  noted,  however,  that 
some  tissues  are  not  in  possession  of  capillary  systems  of 
this  kind,  and  are  nourished  solely  by  lymph.  In  this  group 
of  tissues  belong  the  nails,  hairs,  outer  layer  of  the  skin,  and 
central  area  of  the  cornea  of  the  eye.  It  will  easily  be  seen 


156    THE  CIRCULATION  OF  THE  BLOOD  AND  LYMPH 

that  bloodvessels  would  seriously  impair  the  passage  of  the 
rays  of  light  into  the  interior  of  the  eye. 

The  walls  of  the  veins  are  much  thinner  than  those  of  the 
arteries.  This  structural  difference  is  due  to  the  fact  that 
they  contain  a  very  small  amount  of  muscle  tissue  and  elastic 
fibers,  but  a  large  amount  of  ordinary  connective  tissue. 
On  this  account,  the  caliber  of  the  veins  may  be  made  to 
fluctuate  considerably  in  accordance  with  the  changes  in 
internal  pressure.  Their  walls  collapse  very  readily  when 
this  pressure  is  withdrawn. 

It  is  also  of  interest  to  note  that  the  venous  channels  are 
equipped  with  many  valves  which  are  usually  placed  at  the 
points  where  the  smaller  veins  unite  with  larger  ones.  In 
perfect  agreement  with  the  structure  of  the  lymphatic  valves, 
these  structures  are  composed  of  two  hemispherical  mem- 
branous cups,  the  margins  of  which  are  brought  together 
immediately  if  the  central  pressure  surpasses  that  prevailing 
in  the  more  distal  vessel.  These  valvular  mechanisms 
effectively  oppose  the  backward  displacement  of  the  column 
of  blood  whenever  the  vein  is  compressed  by  an  external 
force.  Thus,  every  contraction  of  the  muscles  must  tend 
to  propel  the  venous  blood  in  the  direction  of  the  heart  and 
not  toward  the  periphery.  Inasmuch  as  the  superficial 
veins  of  the  arms  and  legs  are  more  directly  exposed  to  these 
impacts  than  the  deeper  ones,  they  are  equipped  with  an 
especially  large  number  of  these  valves. 

The  position  of  the  venous  valves  in  the  hand  and  forearm 
may  easily  be  determined  in  the  following  manner:  To  begin 
with,  the  subject  should  hold  his  arm  for  a  few  moments  in 
a  dependent  position,  so  as  to  fill  the  veins  well  with  blood. 
A  piece  of  soft  rubber  tubing  is  then  lightly  wound  around  the 
arm  near  the  elbow.  On  raising  the  arm  to  about  the  level 
of  the  heart,  the  veins  will  be  sharply  outlined  against  the 
integument.  If  one  of  the  smaller  veins  is  now  compressed 
with  the  tip  of  the  index  finger  of  your  right  hand,  while  its 
contents  are  squeezed  into  the  next  collecting  channel  by 
drawing  the  index  finger  of  your  left  hand  gently  across  this 
vein  in  a  direction  from  periphery  to  center,  the  point  of 
entrance  of  the  tributary  vein  will  be  clearly  marked  off  by  a 


THE    FLOW    OF    THE   BLOOD  157 

rounded  eminence.  Obviously,  the  closure  of  the  valve 
situated  at  the  confluence  of  these  veins,  does  not  permit 
the  blood  to  re-enter  the  more  peripheral  segment. 

The  Flow  of  the  Blood.  —  The  chief  purpose  of  the  circula- 
tion is  to  supply  the  different  tissues  with  nutritive  material 
and  to  remove  from  them  their  waste  products.  This  inter- 
change is  accomplished  in  the  capillaries,  where  the  blood 
and  lymph  are  separated  from  one  another  by  only  a  thin 
layer  of  plate-like  cells.  When  considered  from  the  stand- 


S 


A   '•-...  /     c       \ 


/ 


FIG.  69.  —  Diagram  to  illustrate  the  relationship  between  the  size  of  the 
blood-bed  and  the  velocity  of  the  flow.  B,  cross-section;  S,  speed  of  flow 
in  (A)  arteries;  C,  capillaries  and  (V)  veins;  Z,  zero  line. 

point  of  metabolism,  the  arteries  and  veins  are  of  lesser 
importance  than  the  capillaries,  because  they  merely  play  the 
part  of  supply  and  drainage  channels  for  the  latter.  It 
cannot  surprise  us,  therefore,  to  find  that  the  arterial  blood- 
flow  is  very  rapid,  while  that  in  the  capillaries  is  surprisingly 
slow.  Obviously,  since  the  aforesaid  interchanges  take 
place  in  the  tissues,  a  sufficient  time  must  be  allowed  the 
blood  in  which  to  unload  its  nutritive  substances  and  to 
acquire  the  tissue  waste.  In  the  arteries  and  veins,  on  the 
other  hand,  the  speed  of  the  blood  may  well  be  much  in- 
creased, because  practically  no  cellular  interchanges  are 
accomplished  here. 

It  has  been  shown  by  means  of  various  instruments,  that 
the  speed  of  the  bloodflow  is  greatest  in  the  arteries,  least  in 
the  capillaries,  and  intermediate  in  the  veins.  Such  arteries 
as  the  common  carotids  which  supply  the  region  of  the  head, 
are  traversed  with  a  velocity  of  about  250  mm.  in  a  second. 


158        THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

Accordingly,  it  might  be  conjectured  that  the  blood  traverses 
the  body  of  a  person  of  average  height  in  the  course  of  about 
6  to  8  seconds.  It  has  been  stated,  however,  that  the  speed 
of  the  blood  is  markedly  diminished  by  the  friction  en- 
countered by  it  during  its  passage  through  the  capillaries 
and  hence,  a  very  much  longer  time  must  be  required  for  its 
complete  circulation.  The  value  usually  .given  is  30  seconds, 


FIG.  70. — Area  of  capillaries.  Tubules  of  different  diameter  are  shown, 
some  so  small  that  the  red  cells  cannot  enter  at  all  and  others  through 
which  they  can  only  pass  by  assuming  an  elliptical  outline. 

so  that  about  35  heart  beats  are  necessary  in  order  to  drive 
a  droplet  of  blood  from  the  left  ventricle  to  the  right  auricle. 

The  average  velocity  of  the  bloodflow  in  the  capillaries 
is  something  like  1.0  mm.  in  a  second,  and  that  in  the  veins, 
about  60  mm.  in  a  second.  In  explaining  this  sudden  reduc- 
tion in  the  speed  of  the  flow  at  the  capillaries,  brief  reference 
should  first  be  made  to  the  fact  that  the  speed  of  flow  of  a 
river  is  always  greatest  at  the  point  where  the  cross-section 
of  the  river-bed  is  smallest.  Repeated  measurements 
have  proven  that  the  lumen  of  an  artery  is  always  very 


THE    FLOW    OF    THE   BLOOD  159 

much  smaller  than  the  cross-area  of  the  corresponding  system 
of  capillaries.  Thus,  it  may  be  concluded  that  the  cross- 
section  of  all  the  arteries  combined  is  considerably  smaller 
than  that  of  all  the  capillaries.  Consequently,  the  velocity 
of  the  bloodflow  must  be  correspondingly  greater  in  the 
arteries,  because  the  quantity  of  the  circulating  blood  does 
not  change  materially  from  moment  to  moment.  The 
veins  occupy  an  intermediary  position  in  size  as  well  as  in 
the  speed  with  which  the  blood  traverses  them. 

The  circulation  of  the  blood  may  be  observed  directly  by 
placing  certain  translucent  tissues  under  the  ocular  of  a 
microscope.     The  preparations  usually  made  use  of  for  this 
study  are  the  web  and  mesentery  of  the  frog.     Our  attention 
is  first  attracted  by  the  amazingly  large  number  of  tubules 
of  different  caliber  which  one  of  these  small  microscopic 
fields  presents.     Through  some  of  these  the  blood  rushes 
with    such    great  speed  that  its   corpuscular  constituents 
cannot  be  made  out  at  all,  while  others  contain  only  a  few 
slowly  moving  red  cells  at  considerable  distances  from  one 
another.     In  reality,  of  course,  the  bloodflow  is  not  so  rapid, 
because  the  ocular  subjects  the  preparation  to  a  magnification 
of    possibly    fifteen    diameters.     Undoubtedly,    the    most 
interesting  picture  is  presented  by  those  capillaries  which 
possess  a  diameter  somewhat  smaller  than  that  of  the  red 
cells,  so  that  these  elements  cannot  get  through  them  unless 
considerably  compressed  from  side  to  side.     When  observing 
the  places  of  bifurcation  of  some  of  these  capillaries,  one  may 
note  how  the  red  cells  are  thrown  against  the  sharp  edge 
between  them,  and  are  rocked  back  and  forth  a  number  of 
times  before  they  actually  succeed  in  escaping  into  one  or 
the  other  of  the  distal  branches.     The  arterioles  are  easily 
recognized,    because    they    are    larger   in  size  and  possess 
relatively  thick  walls.     The  blood  traverses  them  with  a 
speed  considerably  greater  than  that  observed  in  the  capil- 
laries proper.     Contrariwise,  the  venules  possess  thin  walls, 
and  contain  blood  somewhat  darker  in  color  than  that  of 
the  arterioles.     The  speed  of  the  blood  flow  in  the  venules 
is  considerable,  although  not  quite  so  rapid  as  that  noted 
in  the  arterioles. 


CHAPTER  XV 
BLOODPRESSURE  AND  RELATED  PHENOMENA 

The  Differences  in  Pressure. — It  is  a  well  known  fact  that 
if  the  outer  layer  of  the  skin  is  scraped  off,  a  large  number  of 
bleeding  points  will  appear,  indicating  the  locations  of  the 
opened  vessels.  The  small  drops  of  blood  oozing  out  of  them, 
finally  coalesce  and  cover  the  injured  region  with  a  thin 
coagulum.  If,  however,  the  part  is  incised  more  deeply, 
the  blood  rushes  forth  in  much  greater  volume,  its  manner  of 
escape  becoming  jet-like  when  a  larger  artery  has  been  cut. 
While  an  opened  vein  also  sends  forth  a  large  amount  of 
blood,  the  force  with  which  it  is  ejected  is  by  no  means  con- 
siderable. These  simple  facts  prove  very  clearly  that  the 
blood  is  held  in  the  vascular  system  under  a  certain  pressure, 
commonly  designated  as  bloodpressure.  This  pressure  is 
greatest  in  the  arteries,  intermediate  in  the  capillaries,  and 
least  in  the  veins.  Obviously,  therefore,  the  circulation  of 
the  blood  depends  upon  the  same  factors  as  the  flow  of  water. 
It  is  driven  through  a  series  of  tubes,  meanwhile  constantly 
endeavoring  to  escape  from  the  high  pressure  to  which  it  has 
been  subjected  by  the  action  of  the  heart.  The  pressure 
originally  imparted  to  it  by  this  organ,  is  gradually  used  up  in 
consequence  of  the  resistance  resident  in  the  vascular 
channels. 

The  term  bloodpressure  is  usually  employed  to  designate 
the  pressure  existing  in  the  large  arteries.  This  pressure 
is  actually  somewhat  lower  than  that  prevailing  in  the  left 
ventricle,  but  very  much  higher  than  that  existing  in  the 
capillaries  and  veins.  Supposing  that  the  left  ventricle" 
develops  a  pressure  of  120  mm.  Hg,  we  may  expect  the 
pressure  in  the  aorta  to  be  something  like  115  mm.  Hg, 
and  that  in  the  distal  arteries,  such  as  the  radial  at  the  wrist, 
about  100  mm.  Hg.  The  fact  that  the  pressure  in  two  so 

160 


BLOOD    PRESSURE    AND    RELATED    PHENOMENA  161 

widely  separated  arteries  shows  only  a  relatively  slight 
difference,  clearly  proves  that  the  resistance  encountered 
by  the  blood  in  the  arterial  system  is  inconsiderable.  Hence, 
only  a  comparatively  small  portion  of  the  original  pressure 
is  lost  in  driving  the  blood  as  far  as  the  arterio-capillary 
junction.  Very  different  conditions  are  met  with  in  the 
capillaries.  Because  of  the  enormously  increased  friction, 
these  tubules  must  give  rise  to  a  very  decisive  fall  in  pressure 
or  "loss  in  head,"  as  the  hydraulic  engineer  would  call  it. 
Accurate  measurements  have  demonstrated  that  the  pres- 
sure in  the  true  capillaries  amounts  to  only  about  40  mm. 


FIG.  71. — Diagram  showing  changes  in  pressure  in  the  vascular  system. 
Z,  abscissa  or  zero-line;  P,  curve  of  pressure  (A)  in  arteries  (C)  in  capillaries 
and  (V)  in  veins.  The  greatest  fall  in  pressure  occurs  in  the  capillaries 
in  which  the  resistance  is  greatest. 

It  will  be  seen,  therefore,  that  the  driving  force  imparted 
to  the  blood  by  the  action  of  the  heart,  is  spent  very  largely 
in  overcoming  the  resistance  resident  in  these  fine  tubules. 
This  conclusion  is  substantiated  by  the  fact  that  the  pressure 
in  the  distal  veins  equals  only  about  10  mm.  Hg.  It  becomes 
apparent  immediately  that  this  pressure  is  entirely  inade- 
quate to  force  the  blood  into  the  right  auricle,  the  distance 
still  to  be  covered  amounting  in  many  instances  to  one  meter 
and  more.  This  difficulty  is  overcome  by  the  fact  that  the 
distended  lung  tissue  exerts  a  constant  pull  upon  the  outer 
surfaces  of  the  soft  and  yielding  walls  of  the  central  veins, 
establishing  therein  a  pressure  somewhat  below  the  atmo- 
spheric. Actual  measurements  of  the  pressure  prevailing  in 
the  veins  near  the  right  auricle,  have  yielded  a  value  of  —5  to 
- 10  mm.  Hg.  Accordingly,  it  may  be  said  that  the  blood 
enters  the  greater  or  systemic  circuit  under  a  pressure  of  about 
120  mm.  Hg  and  leaves  it  under  a  pressure  of  about  —5  mm. 
11 


162         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

Hg.  The  greatest  loss  in  pressure  takes  place  in  the  capil- 
laries. Very  similar  conditions  prevail  in  the  lesser  or 
pulmonary  circuit,  although  these  pressures  are  much  lower 
throughout.  Entirely  in  accord  with  the  work  demanded 
of  it,  the  right  ventricle  produces  a  systolic  pressure  of  only 
about  40  mm.  Hg. 

Methods  of  Determining  the  Bloodpressure. — In  order  to 
determine  the  height  of  any  pressure,  it  becomes  necessary 
to  oppose  and  balance  this  pressure  by  a  force  of  known  mag- 
nitude. We  employ  for  this  purpose  as  a  rule  a  column  of 
mercury  which  is  retained  in  a  U-shaped  tube  of  glass. 
This  instrument  which  is  known  as  a  manometer,  is  connected 
by  means  of  rubber  tubing  with  the  artery  in  which  the  pres- 
sure is  to  be  ascertained  (Fig.  72  M) .  The  connecting  tube 


Fro.  72. — Diagram  illustrating  the  indirect  method  of  measuring  blood- 
pressure.  A,  arm  surrounded  by  a  flat  rubber  pouch,  R;  by  means  of  a 
rubber  bulb,  B,  a  pressure  is  set  up  in  this  system  of  tubing  sufficient  to 
compress  the  artery.  This  moment  is  indicated  by  the  manometer  (M). 

is  filled  with  a  solution  of  sodium  carbonate  to  keep  the 
blood  from  coagulating.  As  soon  as  the  clip  has  been  re- 
moved from  the  artery,  the  blood  enters  the  tubing  and  dis- 
places the  mercury  outward  until  the  latter  has  accurately 
balanced  the  pressure.  The  degree  of  displacement  of  the 
mercury,  i.e.,  the  height  to  which  it  rises,  serves  as  the  index 
of  the  internal  pressure.  But  since  the  tube  is  U-shaped, 
the  level  of  the  mercury  must  fall  in  its  central  limb  and  rise 
an  equal  distance  in  its  distal  limb.  Consequently,  the 
height  indicated  by  the  recording  needle  of  the  manometer 
must  be  multiplied  by  two.  The  standard  of  measurement 
is  the  millimeter. 


BLOOD    PRESSURE    AND    RELATED    PHENOMENA 


163 


The  procedure  here  briefly  outlined  constitutes  the  direct 
method  of  registering  the  bloodpressure.  It  is  applicable 
only  to  animals.  Upon  human  beings  we  make  use  of  the 
indirect  method,  the  principle  of  which  is  to  exert  a  known 
outside  force  upon  a  bloodvessel  until  its  lumen  has  been 
completely  obliterated.  It  may  rightly  be  concluded  that 
the  occlusion  of  the  vessel  must 
take  place  at  the  very  moment 
when  the  external  pressure  just 
barely  overcomes  the  internal 
pressure.  Such  an  instrument 
invariably  consists  of  three  parts : 
namely,  a  narrow  pouch  of  rub- 
ber, a  pump  for  the  inflation  of 
the  pouch,  and  a  manometer  for 
the  registration  of  the  pressure 
existing  in  this  system.  The 
artery  usually  selected  for  these 
tests,  is  the  brachial.  The 
pressure  required  to  accomplish 
its  compression,  may  be  measured 
with  the  aid  of  either  an  ordinary 
mercury-manometer  or  a  tension 
spring,  such  as  is  used  upon 
boilers  to  register  the  steam- 
pressure.  The  former  type  of 
instrument  is  designated  as  a 
sphygmomanometer  and  the  latter, 
as  a  sphygmotonometer  (Fig.  73). 

The  principle  underlying  the 
indirect  method  of  ascertaining  the  bloodpressure,  finds  its 
origin  in  the  practice  of  determining  the  arterial  tension 
by  palpation  of  the  radial  artery.  Two  or  three  fingers  are 
usually  used  for  this  purpose,  the  aforesaid  artery  being 
compressed  with  the  more  central  finger  until  the  pulsations 
can  no  longer  be  felt  by  the  more  distal  one.  The  force 
which  is  required  to  obliterate  the  pulse,  serves  as  a  sub- 
jective measure  of  the  pressure  prevailing  within  this  vessel 
and  the  arterial  system  in  general. 


FIG.  73. — Sphygmomano- 
meter of  recent  construction 
(manufactured  by  Green  and 
Bauer) . 


164         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

Palpation. — In  ascertaining  the  bloodpressure  by  means 
of  the  sphygmomanometer,  the  rubber  cuff  takes  the  place 
of  the  central  finger.  It  is  usually  applied  to  the  arm,  and  is 
gradually  inflated  until  the  pulse  at  the  wrist  disappears.  At 
the  very  moment  of  its  disappearance  the  pressure  in  the  arm- 
piece  must  have  just  overcome  the  pressure  within  the 
brachial  artery.  The  value  of  the  latter  is  indicated  by  the 
height  of  the  column  of  mercury.  This  procedure  may  also 
be  reversed  in  the  following  manner:  Knowing  that  the 
bloodpressure  of  a  normal  person  does  not  exceed  140mm. 
Hg,  the  cuff  is  inflated  to  a  point  somewhat  above  this  value. 
Naturally,  the  radial  pulse  is  obliterated  at  this  time.  If 
the  air  is  now  allowed  to  escape  slowly  through  the  detention- 
ing  valve,  a  point  will  eventually  be  reached  when  the  radial 
pulse  again  becomes  palpable.  This  point  indicates  the 
moment  when  the  systolic  arterial  pressure  just  barely 
overcomes  the  outside  pressure.  A  reading  of  the  pressure 
is  taken  at  this  time. 

Auscultation. — It  will  be  shown  later  that  the  bloodpressure 
presents  definite  variations  with  every  contraction  of  the 
heart,  and  every  respiratory  movement.  Concerning  the 
former,  it  should  be  noted  •  at  this  time  that  the  ejection  of 
the  ventricular  contents  raises  the  arterial  pressure  very 
abruptly  to  a  point  about  35  to  40  mm.  Hg  above  that 
prevailing  during  the  diastolic  period  of  the  heart.  In 
other  words,  the  arterial  bloodpressure  exhibits  a  systolic 
maximal  and  a  diastolic  minimal  value  with  every  cardiac 
cycle.  The  difference  between  these  two  values  is  designated 
as  the  pulse-pressure.  Thus,  supposing  that  the  systolic 
pressure  is  120  mm.  Hg  and  the  diastolic  80  mm.  Hg,  the 
pulse-pressure  amounts  to  40  mm.  Hg. 

While  the  procedure  of  palpation  does  not  permit  us  to 
ascertain  anything  more  than  the  systolic  pressure,  all  of  the  _ 
aforesaid  values  may  be  obtained  by  the  method  of  ausculta- 
tion. It  is  a  well  known  fact  that  the  compression  of  a  blood- 
vessel gives  rise  to  a  noise  distally  to  the  point  of  constriction 
which  is  due  to  the  formation  of  whirls  in  the  distorted  column 
of  blood.  Supposing  then  that  the  cuff  is  first  inflated 
beyond  the  bloodpressure  and  is  then  gradually  deflated,  11 


BLOOD    PRESSURE    AND    RELATED    PHENOMENA  165 

point  will  be  reached  when  the  systolic  pressure  breaking 
through  the  constricted  area  produces  a  sound  which,  as 
experience  has  shown,  is  best  heard  at  the  bifurcation  of  the 
brachial  artery  near  the  elbow  joint.  Accordingly,  if  a 
stethophone  is  applied  to  the  ventral  surface  of  this  area  at  a 
distance  of  a  few  centimeters  below  the  edge  of  the  arm- 
piece,  the  systolic  pressure  will  be  indicated  by  very  faint 
sounds  comparable  in  a  measure  to  those  heard  over  the  apex 
of  the  heart.  If  the  cuff  is  deflated  further,  these  sounds 
gradually  increase  in  their  intensity  and  suddenly  disappear 
at  a  point  about  40  mm.  Hg  below  the  systolic  value  of  the 
pressure.  At  the  very  moment  of  their  disappearance  the 
manometer  registers  the  diastolic  pressure.  Hence,  suppos- 
ing that  these  sounds  first  become  audible  at  120  mm.  Hg,  and 
disappear  at  80  mm.  Hg,  the  pulse-pressure  amounts  to 
40  mm.  Hg. 

It  should  also  be  observed  that  the  mercury  in  the  tube  of 
the  manometer  exhibits  certain  oscillations  during  the  defla- 
tion of  the  cuff  which  may  be  used  as  a  guide  in  detecting 
the  points  of  maximal  and  minimal  pressure.  Although 
small  at  first,  these  fluctuations  gradually  increase  in  ampli- 
tude until  the  diastolic  minimum  has  been  reached. 

The  Height  of  the  Arterial  Pressure. — Inasmuch  as  the 
ventricular  pressure  is  the  basis  of  the  circulation  of  the 
blood,  and  inasmuch  as  the  life  of  the  tissue  cells  depends 
upon  a  proper  supply  of  blood,  these  measurements  must 
necessarily  play  a  very  important  part  in  ascertaining  the 
functional  capacity  of  the  body  as  a  whole.  Experience  has 
shown  that  the  normal  bloodpressure  in  men  is  about  120 
mm.  Hg  and  in  women  about  110  mm.  Hg.  Individual 
variations,  however,  are  not  uncommon,  although  it  may  be 
said  that  the  upper  normal  limit  for  men  is  140  mm.  Hg  and 
for  women  130  mm.  Hg.  Pressures  higher  than  these  in- 
variably suggest  some  abnormal  circulatory  condition.  It 
is  to  be  remembered,  however,  that  the  pressure  increases 
constantly  with  age,  owing  chiefly  to  a  diminution  in  the 
elasticity  of  the  vessels.  Thus,  a  pressure  of  140  mm.  Hg 
at  40  years  of  age  is  not  uncommon,  nor  is  one  of  160  mm.  Hg 


166         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

at  sixty  years  of  age.  The  upper  normal  limit  at  this  time 
of  life  is  180  mm.  Hg. 

It  is  true,  however,  that  temporary  rises  and  falls  may  be 
produced  by  various  physiological  means.  Chief  among 
these  is  muscular  exercise.  Thus,  it  is  a  matter  of  common 
experience  that  even  slight  muscular  activity  increases 
the  frequency  of  the  heart  and  causes  the  arterial  pressure  to 
rise  20  to  30  mm.  Hg.  Muscular  exercise  intensifies  the 
circulation,  because  a  larger  quantity  of  blood  must  then 
be  supplied  to  the  working  organs  in  order  to  cover  more 
fully  their  requirement  in  nutritive  material  and  to  remove 
from  them  the  extra  amounts  of  waste.  This  increase  in 
their  metabolism  accounts  also  for  the  greater  respiratory 
activity  invariably  associated  with  muscular  efforts.  But 
these  effects  are  only  temporary  in  their  nature,  and  normal 
conditions  of  pressure  and  flow  are  again  established  within 
a  short  time  after  the  exercise,  provided  it  has  not  been 
severe  nor  unduly  prolonged.  In  the  latter  case,  the 
pressure  may  retain  a  value  somewhat  below  normal  for 
some  time. 

Posture  affects  the  arterial  pressure  in  a  very  characteristic 
manner.  As  a  rule,  the  change  from  the  recumbent  to  the 
standing  position  leads  to  an  increase  in  the  rate  of  the  heart 
which  enables  the  bloodpressure  to  retain  its  normal  value  or 
to  assume  a  level  somewhat  above  normal.  A  marked  fall 
invariably  indicates  a  loss  of  tonus  of  the  bloodvessels, 
allowing  the  influence  of  gravity  to  come  into  play.  It 
is  a  matter  of  common  experience  that  bodily  and  mental 
fatigue,  as  well  as  engorgements  of  the  digestive  organs,  give 
rise  to  a  series  of  peculiar  symptoms,  such  as  dizziness,  un- 
steadiness, and  general  weakness.  The  reason  for  this  must 
be  sought  in  a  deficient  bloodsupply  of  the  brain,  brought 
about  by  the  relaxation  of  the  bloodvessels  in  other  parts  of 
the  body.  The  vessels  chiefly  concerned  in  the  production  of 
these  phenomena  are  those  of  the  portal  organs.  Very 
similar  reactions  are  often  noted  in  persons  who  have  been 
confined  in  bed  for  some  time.  Their  bloodvessels  must  first 
regain  their  normal  tonicity  before  they  will  be  able  to  resist 
the  effects  of  gravity. 


BLOOD    PRESSURE    AND    RELATED    PHENOMENA          167 

Inasmuch  as  the  metabolism  of  all  the  tissues  is  greatly 
diminished  during  sleep,  it  need  not  surprise  us  to  find  that 
the  frequency  of  the  heart,  as  well  as  that  of  respiration, 
retains  during  this  period  a  value  only  about  three-fourths 
of  normal.  For  the  same  reason,  the  bloodpressure  assumes 
at  this  time  a  level  considerably  below  its  usual  one.  These 
effects  have  been  made  use  of  repeatedly  in  explaining  the 
phenomenon  of  sleep,  it  being  held  that  the  cells  composing 
the  higher  centers  enter  this  state  of  depressed  function  in 
consequence  of  a  diminution  in  their  bloodsupply.  Thus, 
it  is  believed  that  the  volume  of  the  brain  decreases  during 
sleep,  because  a  portion  of  the  blood  ordinarily  allotted 
to  this  organ  is  temporarily  diverted  into  the  portal  and 
cutaneous  circuits. 

A  similar  compensation  takes  place  after  meals.  Inasmuch 
as  a  large  amount  of  blood  is  then  needed  by  the  digestive  or- 
gans, the  general  circulatory  channels,  inclusive  of  those  of 
the  brain,  transfer  a  portion  of  their  blood  to  the  portal 
organs.  Consequently,  mental  and  bodily  rest  are  to  be 
recommended  for  some  time  after  the  ingestion  of  a  meal  in 
order  not  to  impair  the  intensity  of  the  digestive  processes. 
Much,  however,  depends  upon  the  quantity  and  quality  of 
the  food  consumed.  Excessive  amounts  of  food  frequently 
augment  the  general  bloodpressure  considerably.  For  this 
reason,  persons  whose  bloodvessels  have  been  rendered 
inelastic  by  calcareous  infiltration,  are  at  this  time  in  parti- 
cular danger  of  suffering  a  rupture  of  one  of  these  brittle 
tubules. 

Variations  in  the  outside  temperature,  such  as  may  be 
produced  by  cold  and  warm  baths,  give  rise  to  very  striking 
changes  in  the  bloodpressure  and  flow.  Since  cold  con- 
stricts the  superficial  vessels,  their  contents  are  temporarily 
diverted  into  the  internal  organs.  Warmth  applied  to  the 
body-surface,  dilates  these  vessels  and  allows  the  blood  to 
enter  the  external  channels.  The  most  efficient  results  are 
obtained  by  bathing  in  water  of  about  34°  C.,  because  this 
temperature  constricts  the  cutaneous  vessels  and  augments 
the  force  of  the  heart  beats,  thereby  intensifying  the  entire 
circulation.  Hot  baths  increase  the  frequency  of  the  heart, 


168         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

but  dilate  the  vessels  in  such  a  measure  that  the  circulation 
cannot  be  augmented. 

Several  of  the  aforesaid  changes  have  been  made  use  of  in 
the  compilation  of  certain  tests  by  means  of  which  the 
physical  condition  of  a  person  may  be  rated.  The  most 
promising  of  these  are  those  devised  by  Foster,  Barrach, 
Crampton,  Barringer,  and  Schneider.1 

The  Arterial  Pulse. — If  the  finger  is  placed  upon  a 
superficial  artery,  such  as  the  radial,  an  intermittent  impulse 
will  be  felt  which  is  due  to  the  sudden  distention  and  subse- 


FIG.  74. — The  character  of  the  arterial  pulse.  AB,  anacrotic  limb; 
BC,  catacrotic  limb;  B,  apex;  D,  dicrotic  wave;  N,  dicrotic  notch;  E, 
predicrotic  wave;  F,  postdicrotic  waves. 


quent  recession  of  the  arterial  wall.  It  need  scarcely  be 
mentioned  that  these  pulsations  originate  in  changes  in  the 
arterial  pressure.  The  high  systolic  pressure  forces  the 
wall  outward,  while  the  diastolic  pressure  permits  it  to 
recoil.  Consequently,  it  may  be  concluded  that  the  cause 
of  the  pulse  lies  in  the  wave  of  high  pressure  produced  in 
the  arch  of  the  aorta  by  the  ejection  of  the  ventricular 
contents.  From  here  this  wave  advances  rapidly  through 
the  smaller  arteries  until  it  is  neutralized  by  the  high  resist- 
ance resident  in  the  capillaries. 

^ee:  Jour  Am.  Med.  Assoc.,  1914,  Ixii,  525  (Barrach);  Med.  News, 
1905,  Ixxxvii,  529  (Crampton);  Arch.  Int.  Med.,  1915,  xvi,  795  (Bar- 
ringer);  Jour.  Am.  Med.  Assoc.,  1920,  Ixxiv,  1507,  and  ibid.,  1921, 
Ixxvi,  705  (Scott). 


BLOOD    PRESSURE    AND    RELATED    PHENOMENA  169 

The  foregoing  discussion  must  lead  us  to  surmise  that  the 
pulsations  in  such  arteries  as  the  radial,  carotid,  and  temporal, 
occur  at  clearly  recognizable  intervals  after  the  corresponding 
systoles  of  the  ventricle.  This  assumption  may  be  proved 
to  be  correct  by  the  simultaneous  palpation  of  the  wall  of 
the  chest  in  the  region  of  the  cardiac  apex  and  of  any  one  of 
the  aforesaid  bloodvessels.  Furthermore,  by  applying  two 
levers  to  a  bloodvessel  at  a  considerable  distance  from  one 
another,  it  can  easily  be  shown  that  the  more  distal  lever  is 


FIG.  75. — The  Dudgeon  sphygmograph  in  position. 

moved  at  an  appreciable  interval  after  the  central  one.  By 
comparing  the  distance  with  the  time,  it  has  been  shown 
that  this  wave  progresses  with  a  velocity  of  7  m.  in  a  second. 
Hence,  its  speed  is  considerably  greater  than  that  of  the 
arterial  bloodstream  which,  as  has  been  stated  above, 
amounts  to  only  25  cm.  in  a  second.  In  order  to  be  able  to 
recognize  these  two  factors  separately,  the  student  is  re- 
minded of  the  fact  that  a  stone  thrown  into  a  river  produces 
ripples  upon  its  surface  which  rapidly  spread  in  all  directions 
from,  the  center  of  the  disturbance.  The  speed  with  which 
these  undulations  advance  is  very  much  greater  than  that 
of  the  flow  of  the  water. 

The  character  of  the  arterial  pulse  is  usually  studied  with 
the  aid  of  an  instrument  which  is  known  as  a  sphygmograph. 
Its  principal  part  is  a  tension  spring  which  is  applied  to  the 


170         THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

region  of  the  artery  and  communicates  its  displacements  to  a 
writing  lever.  As  may  be  surmised,  a  record  of  any  one  of 
these  pulse- waves  exhibits  two  principal  phases:  namely, 
an  upstroke  corresponding  to  the  distention  of  the  vessel, 
and  a  downstroke  indicating  the  subsequent  recoil  of  its 
wall.  It  should  be  noted,  however,  that  the  downstroke 
is  interrupted  by  a  rather  prominent  elevation  which  is 
known  as  the  dicrotic  wavelet. 

Obviously,  inasmuch  as  the  arterial  pulse  serves  as  an 
indication  of  the  frequency  of  the  heart,  it  is  constantly 
employed  by  physicians  in  order  to  obtain  an  idea  regarding 
the  functional  capacity  of  this  organ.  Accordingly,  the 
pulse  may  be  characterized  as  rapid  or  slow,  and  further  more, 
if  note  is  made  of  its  quality,  the  experienced  person  may 
draw  definite  conclusions  regarding  the  tension  prevailing 
in  the  arterial  system.  Thus,  the  pulse  is  said  to  be  soft 
when  the  pressure  is  relatively  low  and  the  vessels  possess 
a  proper  degree  of  elasticity.  A  hard  pulse,  on  the  other 
hand,  indicates  a  high  bloodpressure  and  relative  inelasticity 
of  the  vessels.  The  pulse  is  also  described  as  small  or  large, 
these  terms  referring  more  particularly  to  its  volume  or 
magnitude. 

In  this  connection  brief  reference  should  also  be  made 
to  the  fact  that  the  central  veins  pulsate  synchronously 
with  the  heart  beat.  The  cause  of  this  venous  pulse  lies  in  the 
fact  that  the  changes  in  pressure  occurring  in  the  auricles, 
are  propagated  outward  into  the  veins.  It  has  been  stated 
above  that  the  venous  orifices  of  the  heart  are  not  guarded 
by  valves  and  allow  a  free  communication  with  the  veins 
even  during  the  contraction  of  the  auricles.  A  very  con- 
venient region  for  observing  these  pulsations  is  the  jugular 
fossa,  in  the  depth  of  which  is  embedded  the  central  portion 
of  the  external  jugular  vein. 


CHAPTER  XVI 

THE    NERVOUS    CONTROL    OF   THE    HEART    AND 
BLOODVESSELS 

A.  THE  REGULATION  OF  THE  BEAT  OF  THE  HEART 

The  Excised  Heart. — It  is  a  well  known  fact  that  the 
heart  may  continue  its  activity  even  after  it  has  been  isolated 
from  the  central  nervous  system  by  severing  all  of  its  nerves. 
In  fact,  the  excised  heart  of  the  lower  animals  will  beat  rhyth- 
mically for  many  days  and  even  weeks  if  placed  in  a  dish 
under  favorable  conditions  of  moisture  and  temperature. 
Very  similar  results  may  be  obtained  with  the  hearts  of  the 
mammals,  but  since  these  organs  possess  a  smaller  storative 
power,  they  must  be  supplied  with  a  nutritive  solution  of 
some  kind  in  order  to  enable  them  to  continue  their  activity 
outside  the  body.  Lastly,  it  is  possible  to  excise  small 
strips  of  the  ventricle  and  to  activate  these  preparations  by 
immersing  them  in  solutions  of  certain  salts,  for  example, 
those  of  the  chlorids  of  sodium,  calcium,  and  potassium. 
Under  favorable  conditions  these  pieces  of  cardiac  muscle  will 
continue  to  beat  rhythmically  for  many  hours  and  even  for 
days. 

These  facts  clearly  prove  that  the  activity  of  the  heart 
is  not  dependent  upon  the  central  nervous  system,  and  that 
the  stimulus  to  contract  is  brought  to  bear  directly  upon  the 
tissues  composing  this  organ.  In  this  regard,  cardiac  muscle 
tissue  differs  materially  from  skeletal  muscle,  because  every 
contraction  of  the  latter  is  instigated  by  impulses  from  the 
higher  nerve  centers.  Accordingly,  it  may  be  concluded 
that  the  central  nervous  system  merely  serves  the  purpose  of 
correlating  the  automatic  action  of  the  heart  with  those  of 
other  tissues  and  organs.  It  is  also  a  matter  of  common 
experience  that  the  activity  of  this  central  pumping  mechan- 

171 


172    THE  CIRCULATION  OF  THE  BLOOD  AND  LYMPH 


ism  may  be  markedly  altered  at  any  time  by  stimuli  arising 
in  other  parts  of  the  body,  i.e.,  in  a  reflex  manner.  The 
results  jthat  may  be  obtained  in  this  way,  are  either  a  quicken- 
ing or  a  slowing  of  its  beat.  The  former  effect  is  termed 

cardio-acceleration    and   the   latter, 

cardio-inhibition . 

The     Cardiac     Nerves. — Having 

established  this  important  fact,  let 

us  endeavor  to  trace  the  nervous 

: connections  by  means  of  which  the 
aforesaid  impulses  from  the  central 
nervous  system  are  conveyed  to 
the  heart.  Possibly  the  simplest 
anatomical  relationship  exists  in  the 
frog.  We  find  here  that  the  heart 
receives  two  sets  of  nerve  fibers,  .one 
being  derived  from  the  vagi  nerves, 
constituting  the  tenth  pair  of  cranial 
nerves,  and  one  from  the  sympa- 
thetic ganglia  situated  along  the 
thoracic  division  of  the  spinal  cord. 
It  is  entirely  probable  that  both 

FIG.  76. — Schema  to  show  <•    r-i  •    •  •  .    • 

the  course  of  the  cardiac  sets  of  nbers  originate  in  certain 
nerves  in  the  frog.  A,  vagai  nerve  cells  which  occupy  a  part  of 
the  enlarged  upper  portion  of  the 
spinal  cord  or  medulla  oblongata. 
This  group  of  cells  forms  the  so- 
called  cardiac  center.  It  communi- 
cates with  different  afferent  chan- 
nels through  which  other  parts  of 

the  body  are  enabled  to  influence  its  activity.  In  this 
manner  its  motor  action  may  be  varied  in  favor  of  either 
cardio-acceleration  or  cardio-inhibition.  The  nature  of  the 
impulses  received  determines  the  character  of  the  reaction 
that  is  required  by  the  other  organs  to  conform  to  their 
state  of  activity. 

The  efferent  impulses  discharged  by  the  cardiac  center 
reach  the  heart  by  way  of  two  separate  channels.  Those 
intended  to  slow  its  beat  descend  through  the  vagi  nerves, 


fibers  are  still  separate;  B, 
sympathetic  fibers  are  still 
separate;  C,  both  types  of 
fibers  have  combined  to 
form  the  vagosympathetic 
nerve.  R,  Remak's  gan- 
glion; B,  Bidder's  ganglion. 


CONTROL  OF  THE  HEART  AND  BLOODVESSELS     173 

and  those  purposing  to  increase  its  frequency  through  the 
spinal  cord  and  sympathetic  system  of  the  thorax.  Near 
the  heart  these  motor  fibers  ramify  extensively  to  form  a 
plexus,  whence  they  are  distributed  to  the  pace-maker  and 
ganglia  within  this  organ.  This  arrangement  is  also  clearly 
in  evidence  in  the  mammals,  although  anatomically  more 
difficult  to  trace  than  in  the  amphibians  and  reptiles. 

Inhibition  of  the  Heart. — If  the  apex  of  the  heart  of  a 
recently-killed  frog  is  connected  by  means  of  a  thread  with  a 
writing  lever,  properly  arranged  to  register  its  excursions 
upon  the  paper  of  a  kymograph,  a  record  will  be  obtained 
such  as  is  represented  in  Fig.  77.  Provided  the  suspension 


FIG.  77. — Record  of  the  contractions  of  the  frog's  heart.      The  time  is 
registered   in   seconds. 

method  is  employed,  the  upstrokes  of  the  lever  indicate  the 
systolic  phases  of  the  ventricle,  and  the  downstrokes  its 
diastolic  periods.  Various  procedures  may  at  this  time  be 
followed  in  order  to  determine  their  influence  upon  the  action 
of  the  heart.  A  very  simple  experiment  is  to  allow  a  few 
drops  of  warm  or  cold  saline  solution  to  flow  over  its  surface. 
It  will  be  noted  that  warmth  increases  the  frequency  of  its 
beat,  while  cold  diminishes  it.  Likewise,  we  may  then  isolate 
and  stimulate  the  aforesaid  motor  nerves  in  an  attempt  to 
analyze  their  action  in  greater  detail.  When  graphically 
portrayed  in  this  way,  the  excitation  of  the  vagus  with  a 
moderately  strong  current  usually  yields  a  record  such  as  is 
represented  in  Fig.  78. 

It  will  then  be  observed  that  the  heart  ceases  to  beat  a 
few  moments  after  the  application  of  the  current,  its  inhibi- 


174         THE    CIRCULATION    OF    THE   BLOOD    AND    LYMPH 

tion  being  indicated  in  the  record  by  the  straight  line. 
Furthermore,  as  the  heart  becomes  more  and  more  diastolic, 
it  gradually  acquires  a  larger  amount  of  blood.  Complete 
inhibition,  of  course,  means  a  highly  distended  organ  and 
cessation  of  the  circulation.  The  arterioles  empty  them- 
selves slowly  by  transferring  their  contents  into  the  larger 
veins.  This  condition  is  practically  identical  with  that 
established  shortly  after  death,  when  the  recoil  of  the  arterial 
walls  drives  the  blood  into  the  central  veins  and  right  side  of 
the  heart. 


FIG.  78. — Record  of  the  contractions  of  the  frog's  heart  during  stimula- 
tion of  the  vagus  nerve.  The  time  is  given  in  seconds,  the  stimulation 
is  indicated  by  the  signal. 

It  is  well  to  remember,  however,  that  a  heart  cannot  be 
permanently  inhibited  by  the  stimulation  of  the  vagus  nerve. 
Curiously  enough,  it  again  commences  to  beat  after  a  few 
minutes  of  inhibition  and  then  continues  its  activity  in  spite 
of  the  continued  application  of  the  electrical  current.  This 
phenomenon  is  called  " escape  from  inhibition." 

Acceleration  of  the  Heart. — Contrary  to  the  excitation  of 
the  vagus  nerve,  the  stimulation  of  the  sympathetic  gives 
rise  to  an  appreciable  increase  in  the  frequency  and  force  c£ 
the  cardiac  contractions.  It  may  be  said,  therefore,  that 
these  nerves  are  functionally  antagonistic  to  one  another. 
This  fact  is  well  substantiated  by  the  changes  following  the 
division  of  the  vagi  nerves,  because  very  shortly  after  the 
destruction  of  these  paths  the  rate  of  the  heart  is  markedly 
increased.  It  may  be  concluded,  therefore,  that  this  organ 


CONTROL  OF  THE  HEART  AND  BLOODVESSELS     175 

is  ordinarily  held  in  check  by  a  constant  stream  of  minimal 
inhibitory  impulses.  Now,  since  the  division  of  the  vagi 
nerves  destroys  the  path  by  means  of  which  these  restraining 
impulses  are  conducted  to  the  heart,  the  frequency  of  this 
organ  must  be  considerably  increased  after  this  procedure. 
This  effect  is  considerably  augmented  by  the  pure  accelerator 
impulses  which  reach  the  heart  by  way  of  the  sympathetic 
nerve.  The  latter  then  gain  full  control  over  this  organ. 
We  note  here  a  certain  similarity  between  the  action  of  the 
heart  and  that  of  a  horse  in  harness.  The  horse  represents 
a  self-active  mechanism  which  is  directed  along  definite 
channels  by  means  of  reins.  When  the  latter  are  slackened, 
the  horse  quickens  its  pace  in  a  measure  to  conform  more 
closely  to  its  own  inherent  power. 

B.  THE  REGULATION   OF  THE  CALIBER   OF  THE 
BLOODVESSELS 

The  Vasomotor  Mechanism. — We  have  previously  noted 
that  the  walls  of  the  arteries  adapt  themselves  in  a  perfectly 
passive  way  to  the  changes  in  bloodpressure,  giving  rise  to 
what  is  known  as  the  pulse.  But  besides  these  changes 
which  are,  so  to  speak,  forced  upon  them  by  an  outside 
factor,  the  arterial  wall  is  also  able  to  execute  movements  of 
an  active  kind  in  actual  antagonism  to  the  internal  pressure. 
The  elements  chiefly  concerned  in  this  process  are  the  smooth 
muscle  cells  which,  as  has  been  stated  above,  are  most  numer- 
ous in  the  smaller  arteries  and  are  arranged  here  in  the  form 
of  a  heavy  layer  circularly  around  the  lumen  of  the  vessel. 
Accordingly,  their  contraction  must  diminish  the  size  of  the 
channel  and  thereby  materially  hinder  the  onward  movement 
of  the  blood. 

This  motor  mechanism  is  under  the  control  of  a  special  set 
of  nerves  which  are  known  as  vasomotor  nerves.  In  com- 
plete agreement  with  the  arrangement  of  the  cardiac  center, 
these  nerve  fibers  originate  in  a  colony  of  ganglion  cells 
which  are  situated  in  the  medulla  oblongata,  and  comprise 
the  so-called  vasomotor  center.  This  particular  segment 
of  the  central  nervous  system,  therefore,  controls  the  most 
vital  functions  of  our  body:  namely,  the  action  of  the  heart, 


176    THE  CIRCULATION  OF  THE  BLOOD  AND  LYMPH 

the  size  of  the  arterial  channels  and,  as  will  be  brought  out 
in  a  later  chapter,  the  movements  of  respiration.  It  need 
not  surprise  us,  therefore,  to  find  that  its  destruction  termi- 
nates life  immediately,  because  the  aforesaid  vital  functions 
are  thereby  made  to  cease.  By  analogy  it  may  then  be 
concluded  that  the  destruction  <of  the  brain,  and  more 
especially  of  the  cerebral  cortex,  need  not  prove  fatal  as 
long  as  the  medulla  is  left  intact.  In  the  absence  of  the 
cerebral  cortex  the  animal  simply  follows  a  non-psychic  or 
reflex  life  without  being  deprived  of  its  cardiac,  respiratory, 
and  vasomotor  activities. 

Constriction  and  Dilatation  of  the  Bloodvessels. — The 
vasomotor  center  communicates  by  means  of  diverse  afferent 
paths  with  practically  all  the  sense-organs  of  the  body. 
Consequently,  its  activity  must  undergo  certain  variations 
in  accordance  with  the  character  of  the  impulses  received 
by  it.  The  changes  to  which  it  may  give  rise  are  of  two 
kinds,  namely,  vasoconstriction  and  vasodilatation.  The 
former  phenomenon  consists  in  a  decrease  in  the  size  of  the 
lumen  of  the  bloodvessel  and  the  latter,  in  an  enlargement 
of  its  lumen. 

The  walls  of  the  bloodvessels  usually  retain  a  position 
intermediate  between  constriction  and  dilatation.  This 
statement  should  lead  us  to  infer  that  they  are  ordinarily 
held  in  a  state  of  tonus,  akin  to  that  displayed  by  striated 
muscle.  It  may, .  therefore,  be  surmised  that  the  smooth 
muscle  cells  of  the  arterial  wall  are  the  recipients  of  a  series 
of  subminimal  impulses  which  tend  to  keep  them  in  a  state 
of  functional  alertness  and  efficiency.  But  this  state  of  tonus 
may  be  varied  at  any  time  in  either  direction  by  impulses  of 
greater  strength  and  appropriate  character. 

The  manner  in  which  vasoconstriction  is  brought  about,  is 
easily  understood  if  it  is  remembered  that  by  far  the  largest 
number  of  these  muscle  cells  is  arranged  circularly  around 
the  lumen  of  the  vessel.  Clearly,  the  contraction  of  these 
elements  must  diminish  the  size  of  the  channel  and  retard 
the  escape  of  the  arterial  blood  into  the  capillaries.  Possibly 
the  simplest  way  of  explaining  vasodilatation  is  to  assume  that 
the  tonus  impulses  to  the  bloodvessels  are  inhibited.  The 


CONTROL  OF  THE  HEART  AND  BLOODVESSELS    177 

walls  of  the  bloodvessels  are  thereby  allowed  to  relax  and 
to  be  forced  outward  by  the  internal  pressure.  Naturally, 
such  an  enlargement  of  the  lumen  of  the  arteriole  must 
lower  the  peripheral  resistance,  and  permit  a  larger  quantity 
of  arterial  blood  to  enter  the  capillaries. 

Different  Vasomotor  Reactions. — A  phenomenon  familiar 
to  practically  everybody  is  that  of  blushing.  It  is  a  local 
modification  of  the  circulation  which  is  usually  confined  to  the 
bloodvessels  of  the  cheeks,  but  may  also  involve  a  much 
larger  area.  In  the  light  of  the  preceding  discussion,  it  will 
now  be  evident  that  this  change  is  dependent  upon  a  relaxa- 
tion and  dilatation  of  these  particular  bloodvessels  in  con- 
sequence of  certain  psychic  impressions.  The  impulses 
generated  in  the  higher  centers  are  relayed  to  the  vasomotor 
center,  whence  they  are  distributed  to  the  distal  arteries, 
causing  them  to  dilate.  In  consequence  of  the  inrush  of  a 
larger  amount  of  blood,  the  part  so  affected  becomes  red  in 
color  and  warm  to  the  touch. 

The  reverse  change,  namely,  that  of  paling,  is  usually 
preceded  by  extreme  terror  or  rage.  At  this  time,  the 
bloodvessels  constrict  and  hinder  the  influx  of  blood  into 
these  channels.  The  part  so  affected  loses  its  red  color,  and 
becomes  colder  to  the  touch. 

The  application  of  heat  and  cold  produces  similar  reactions. 
Warmth  relaxes  the  vessels,  while  cold  constricts  them.  In 
some  instances,  however,  this  primary  change  is  soon  fol- 
lowed by  one  of  opposite  character.  Thus,  if  we  pass  from  the 
cold  outside  air  into  a  warm  room,  the  previously  constricted 
bloodvessels  of  the  exposed  parts  of  our  body  relax  and  do 
not  reassume  their  normal  vascular  tone  until  some  time 
later.  The  skin  then  feels  excessively  hot. 

Brief  reference  should  also  be  made  at  this  time  to  the 
fact  that  vasodilatation  favors  the  escape  of  the  body-heat, 
because  the  exposure  of  a  large  amount  of  blood  to  the  colder 
medium,  whether  air  or  water,  greatly  augments  heat  radia- 
tion. For  this  reason,  great  care  should  be  exercised  at  all 
times  to  protect  the  body  against  a  fall  in  its  temperature 
below  its  normal  value  of  37.1°  C.  (98.4°  F.).  A  severe 
"chilling"  of  the  body  usually  causes  it  to  lose  its  vasomotor 
12 


178         THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

tone  and  resistance  against  bacterial  invasion,  Hence,  a 
person  who  has  engaged  in  muscular  exercise  and  whose  cu- 
taneous vessels  are  well  filled  with  blood,  should  safeguard 
his  body  against  excessive  heat  dissipation  by  putting  on 
additional  clothing. 

Another  very  important  reaction  of  this  kind  takes  place 
in  all  glands  whenever  they  are  called  upon  to  furnish  an 
extra  quantity  of  secretion.  Activity  is  invariably  associated 
with  vasodilatation,  because  a  larger  amount  of  blood  must 


FIG.  79. — Schema  illustrating  the  nerve  supply  of  the  submaxillary 
gland.  SG,  submaxillary  gland;  supplied  by  a  small  artery  from  the 
carotid  system  (CA).  It  is  drained  by  a  small  vein  which  generally 
enters  the  facial  (FV)  at  its  point  of  confluence  with  the  lingual  vein  (LV). 
The  external  (ESV)  and  internal  (JSV)  maxillary  veins  invest  the 
gland  and  unite  to  form  the  external  jugular  vein  (EJV).  The  sym- 
pathetic nerve  supply  is  derived  from  the  sup.  cerv.  ganglion  (<SCG). 
The  chorda  tympani  (CT)  attaches  itself  to  the  lingual  nerve  LN  and 
then  to  Wharton's  duct  (W) ;  S,  lower  jaw. 

be  brought  to  these  parts  in  order  to  place  them  in  the  most 
favorable  position  to  increase  their  product.  While  these 
changes  are  clearly  in  evidence  in  all  those  glands  which 
furnish  the  digestive  juices,  a  particularly  interesting  arrange- 
ment is  present  in  the  salivary  glands.  These  organs  receive 
a  twofold  nerve  supply :  namely,  one  directly  from  the  brain 
and  the  other  from  the  sympathetic  ganglia  of  the  thorax. 
Peculiarly  enough,  these  nerves  possess  an  antagonistic 
action  upon  the  bloodsupply  of  these  organs.  If  we  select 
the  submaxillary  gland  as  an  example,  it  will  be  found  that 
its  cerebral  nerve  or  chorda  tympani  possesses  a  vasodilator 
action,  while  the  sympathetic  subserves  vasoconstriction. 
Hence,  it  cannot  surprise  us  to  find  that  the  excitation  of  the 


CONTROL  OF  THE  HEART  AND  BLOODVESSELS     179 

former  nerve  causes  the  aforesaid  gland  to  redden  and  to 
become  warm  to  the  touch,  whereas  the  stimulation  of  the 
latter  causes  it  to  grow  pale  and  colder.  Furthermore,  while 
the  excitation  of  the  chorda  tympani  gives  rise  to  a  very 
copious  flow  of  a  watery  type  of  saliva,  the  excitation  of  the 
sympathetic  yields  only  a  very  moderate  amount  of  very 
thick  saliva. 

The  bloodsupply  of  the  abdominal  organs  is  regulated  by 
the  greater  splanchnic  nerves.  Inasmuch  as  these  nerves 
contain  vasoconstrictor  as  well  as  vasodilator  fibers  and 
control  about  one-fourth  of  the  total  amount  of  blood  present 
in  the  body,  they  are  in  the  best  possible  position  to  exert 
a  very  powerful  influence  upon  the  contents  of  other  circula- 
tory systems.  Reference  has  already  been  made  to  the 
fact  that  the  dilatation  of  the  splanchnic  vessels  permits  a 
certain  quantity  of  blood  to  be  transferred  from  the  general 
circuits  of  the  body  into  those  of  the  kidneys,  stomach, 
intestine,  pancreas,  spleen,  and  liver.  Contrariwise,  the 
constriction  of  these  vessels  causes  a  certain  portion  of  their 
contents  to  be  diverted  into  the  general  bloodvessels,  thereby 
raising  the  systemic  bloodpressure. 

Another  very  important  vasomotor  mechanism  is  the  one 
enabling  the  heart  to  diminish  the  general  bloodpressure. 
While  this  organ  is  able  to  adjust  the  frequency  and  force  of 
its  beat  to  the  arterial  pressure,  it  may  also  happen  at  times 
that  the  resistance  which  it  must  overcome,  is  so  great  that 
it  cannot  act  against  it  without  excessive  strain.  Such  an 
unusual  resistance  is  commonly  established  in  consequence 
of  far-reaching  vasoconstrictions,  and,  naturally,  a  heart 
which  has  been  weakened  by  disease,  is  thereby  placed  in  a 
position  inviting  injury.  It  possesses,  however,  a  safety 
device  in  the  shape  of  certain  afferent  fibers  which  are  con- 
nected with  the  cardiac  and  vasomotor  centers  and  through 
which  a  general  vasodilatation  and  fall  in  bloodpressure 
may  be  occasioned.  This  reflex  is  evoked  whenever  the 
pressure  within  the  arch  of  the  aorta  reaches  a  dangerous 
height.  The  resulting  dilatation  of  the  bloodvessels  and  fall 
in  pressure  immediately  enables  the  heart  to  act  with  much 
greater  freedom. 


180        THE    CIRCULATION    OF    THE    BLOOD    AND    LYMPH 

The  Purpose  of  Vasomotor  Activity. — The  principal  pur- 
pose of  vasomotor  activity  is  to  adjust  the  distribution  of  the 
blood  in  such  a  way  that  almost  any  part  of  the  body  may 
control  its  own  supply  without  necessitating  a  general  change 
in  the  bloodflow  through  other  organs.  In  other  words,  it 
permits  a  certain  change  in  the  distribution  of  the  blood  in 
favor  of  particular  circuits  quite  independently  of  the  action 
of  the  heart.  Many  examples,  however,  could  be  cited  to 
show  that  these  actions  may  also  pursue  a  harmonious 
course.  Thus,  vigorous  exercise  not  only  augments  the 
activity  of  the  heart,  but  also  increases  the  vascular  tonus, 
thereby  quickening  the  entire  circulation. 

The  student  should  also  be  cautioned  at  this  time  not  to 
confound  the  local  effects  of  vasodilatation  with  the  condition 
of  congestion,  because  the  latter  usually  requires  for  its 
development  an  obstruction  to  the  venous  return  and  a  pas- 
sive engorgement  of  the  bloodvessels  situated  centrally  to 
the  block.  It  is  to  be  noted,  therefore,  that  while  the  con- 
gested part  contains  an  unusually  large  quantity  of  blood, 
the  flow  through  it  is  considerably  diminished.  For  this 
reason,  the  blood  so  stagnated  soon  loses  a  large  part  of  its 
oxygen,  and  assumes  a  much  darker  color  than  that  exhibited 
by  the  blood  in  neighboring  parts.  The  tissues  containing 
the  stagnated  blood  assume  a  dark  blue  appearance.  It 
should  be  remembered,  however,  that  this  condition  need 
not  be  localized,  but  may  also  involve  much  larger  regions 
of  the  body,  preeminently  the  venous  system.  It  is  a  well 
known  fact  that  those  lesions  of  the  cardiac  valves  which 
seriously  hinder  the  inflow  of  blood  into  the  right  auricle 
usually  give  rise  to  a  congestion  of  practically  all  the  abdomi- 
nal organs.  This  congestion  is  first  noted  in  the  liver  and 
spleen,  because  these  organs  are  soft  in  texture  and  quickly 
react  to  the  increased  venous  pressure  by  increasing  their 
volume. 


PART  III 
RESPIRATION 

CHAPTER  XVII 
THE  ELEMENTARY  LUNG 

General  Discussion. — In  its  widest  sense  the  term  respira- 
tion is  employed  to  designate  the  interchange  of  the  respira- 
tory gases  between  the  organism  and  the  medium  in  which  it 
is  contained.  Accordingly,  the  process  of  respiration  is 
essentially  a  chemical  one,  because  it  supplies  the  cells  with 
oxygen  and  removes  from  them  the  waste  gas,  carbon  dioxid. 
It  has  previously  been  pointed  out  that  every  cell  gives  rise 
to  energy  which  is  derived  from  the  food,  but  in  order  that  the 
cell  may  be  able  to  accomplish  this  end,  it  must  reduce  the 
nutritive  substances  into  their  simplest  components  by  a 
process  which  requires  the  presence  of  free  oxygen.  Natu- 
rally, considerable  amounts  of  this  gas  are  taken  in  with  the 
food,  but  the  protoplasm  of  the  cell  is  quite  unable  to  utilize 
it  when  firmly  united  with  other  elements  to  form  com- 
pounds. Hence,  the  oxygen  must  be  presented  to  it  in  an 
available  form,  otherwise  it  cannot  be  used  to  oxidize  the 
nutritive  substances. 

While  it  is  evident  that  the  cell  burns  up  materials  in 
the  presence  of  oxygen  and  liberates  carbon  dioxid,  this 
chemical  process  could  not  be  effected  without  certain 
mechanical  procedures.  Respiration,  therefore,  also  pre- 
sents a  mechanical  aspect,  embracing  the  transportation  of 
the  respiratory  gases  through  the  lungs  and  the  blood. 
These  processes  are  usually  considered  under  the  heading  of 
the  mechanics  of  respiration. 

181 


182  KESPIRATION 

Diffusion  Pressure. — The  atmospheric  air  is  a  mixture  of 
gases  comprising  the  following  constituents: 

Oxygen 20 . 94  per  cent. 

Nitrogen 78 . 40  per  cent. 

Argon,  krypton,  neon 0. 63  per  cent. 

Carbon  dioxid 0 . 03  per  cent. 

This  medium  rests  upon  every  area  of  the  surface  of  our  body 
with  a  certain  pressure  which  is  designated  as  atmospheric 
pressure.  At  this  particular  latitude  and  altitude,  it  is 
capable  of  supporting  a  column  of  mercury  760  mm.  in  height. 
It  becomes  less  at  a  higher  level  and  greater  at  a  lower 
in  accordance  with  the  thickness  of  the  layer  of  the  air. 
This  "line"  of  atmospheric  pressure,  equalling  760  mm.  Hg, 
serves  as  the  abscissa  for  all  our  records  of  pressure.  Thus, 
when  we  say  that  the  blood  circulates  under  a  pressure  of  120 
mm.  Hg,  we  wish  to  convey  the  idea  that  this  pressure  exceeds 
that  of  the  atmosphere  by  120  mm. 

It  is  to  be  noted  especially  that  the  atmospheric  pressure 
represents  the  sum  of  the  pressures  exerted  by  its  several 
constituents.  In  other  words,  each  component  of  the  air 
contributes  its  share  toward  the  total  pressure  in  accordance 
with  its  volume.  Accordingly,  since  oxygen  constitutes 
about  one-fifth  of  the  total  volume  of  the  air,  the  pressure 
exerted  by  it  must  amount  to  J£  of  760mm.  Hg,  or  152  mm. 
Hg.  Quite  similarly,  since  fresh  air  is  practically  free  from 
carbon  dioxid,  the  partial  pressure  of  this  gas  in  the  atmos- 
phere must  be  very  close  to  zero. 

Diffusion. — It  is  a  well  known  fact  that  currents  in  air 
arise  whenever  areas  of  unequal  atmospheric  pressure  are 
established.  While  these  differences  persist,  the  air  con- 
tinues to  move  from  the  place  of  high  pressure  to  the  place 
of  low  pressure.  This  movement  ceases  as  soon  as  an  equally 
nation  of  the  pressures  has  been  attained.  Quite  similarly, 
any  constituent  of  the  atmospheric  air  may  be  made  to  move 
independently  of  the  others  by  subjecting  it  to  differences 
in  its  partial  pressure.  Thus,  if  we  bring  two  mixtures  of 
gases  together,  containing  unequal  amounts  of  oxygen,  the 
molecules  of  this  gas  must  continue  to  leave  the  one  embrac- 


THE    ELEMENTARY    LUNG  183 

ing  it  in  larger  quantity  until  its  partial  pressure  has  been 
equalized  in  the  two  places. 

We  know  that  an  organism  uses  up  oxygen  continually 
and  liberates  carbon  dioxid.  Consequently,  the  partial 
pressure  of  the  oxygen  must  be  greater  in  the  medium  than 
in  the  organism,  whereas  that  of  the  carbon  dioxid  must  be 
greater  within  than  without.  In  accordance  with  the  pre- 
ceding discussion,  it  may  then  be  concluded  that  the  mole- 
cules of  oxygen  must  enter  the  organism,  while  those  of 
carbon  dioxid  must  leave  it.  The  cell  wall  does  not  greatly 
impede  the  progress  of  these  molecules,  because  the  differences 
in  the  partial  pressures  of  the  aforesaid  gases  are  more  than 
sufficient  to  overcome  this  obstacle.. 

The  Development  of  the  Simple  Lung. — It  need  scarcely 
be  emphasized  that  the  differences  in  the  partial  pressures 
of  the  respiratory  gases  are  sufficient  to  establish  an  adequate 
interchange  in  all  those  organisms  which  consist  of  only  a 
limited  number  of  cells.  Contrariwise,  it  may  be  conjec- 
tured that  this  process  of  direct  diffusion  must  be  quite 
inadequate  if  the  body  embraces  many  millions  of  cells  and 
is  enveloped  by  a  relatively  impermeable  integument.  In 
order  to  overcome  this  difficulty,  the  surface  of  the  body  has 
been  indented  in  the  form  of  a  sac  filled  with  air.  This 
membranous  pouch  represents  the  elementary  lung.  In 
the  h'gher  animals  it  is  suspended  in  the  fore  part  of  the 
body  cavity  and  communicates  with  the  outside  through  a 
narrow  passage  which  is  termed  the  trachea. 

The  distant  cells  of  the  tissues  are  brought  in  diffusion- 
relation  with  the  air  in  this  pouch  through  the  medium  of  the 
blood.  While  traversing  the  capillary  networks  of  the  lungs 
the  blood  takes  up  oxygen  and  rids  itself  of  a  certain  portion 
of  its  carbon  dioxid.  It  then  rushes  to  the  tissues,  where  it 
transfers  a  part  of  its  oxygen  to  the  cells  and  acquires  carbon 
dioxid.  This  interchange  is  repeated  again  and  again.  It 
will  be  seen,  therefore,  that  the  process  of  respiration  con- 
sists in  reality  of  two,  namely,  a  diffusion  between  the 
air  in  the  lung  and  the  elements  of  the  blood,  and  a  diffu- 
sion between  the  latter  and  the  cells  of  the  different  tissues. 
The  former  is  called  external  respiration  and  the  latter, 


184 


RESPIRATION 


internal  respiration.     It  is  to  be  noted,  however,  that  both 
are  based  upon  the  principle  of  diffusion. 

The  Necessity  of  Breathing. — It  need  scarcely  be  men- 


i 


FIG.  80. — Median  sagittal  section  through  the  head  and  neck,  a, 
superior  meatus  of  the  nose;  b,  middle  meatus  of  the  nose;  c,  inferior 
meatus  of  the  nose ;  d,  torus  tularius  (eustachian  cushion) ;  e,  orifice  of 
auditory  tube;  /,  palatoglossal  fold;  g,  tonsil;  h,  galatopharyngeal  fold; 
k,  aryepiglottic  fold;  I,  ventricle  of  the  larynx;  m,  vocal  cord;  w,  vestibule 
of  nose.  (Radasch.) 

tioned  that  the  continued  movements  of  the  molecules  of 
oxygen  into  the  blood  and  of  the  molecules  of  carbon  dioxid 
into  the  air  of  the  lung  would  eventually  lead  to  an  equaliza- 


THE    ELEMENTARY    LUNG  185 

tion  of  the  diffusion  pressures  and  a  cessation  of  this  inter- 
change. In  other  words,  it  would  cause  the  death  of  the 
organism  by  asphyxia.  Hence,  in  order  to  retain  the  tissues 
in  an  aerated  condition,  it  is  absolutely  necessary  to  maintain 
the  differences  in  the  partial  pressures  of  these  gases,  and 
this  end  can  only  be  accomplished  by  the  constant  renewal 
of  the  air  in  the  lung.  This  statement  furnishes  the  principal 
reason  for  the  act  of  breathing,  the  mechanics  of  which 
form  one  of  the  most  instructive  and  interesting  chapters  in 
physiology. 

The  principle  upon  which  the  renewal  of  the  air  in  the 
lungs  is  based,  is  a  very  simple  one,  and  may  be  imitated 
very  easily  with  the  aid  of  a  pair  of  bellows.  When  com- 
pressed, the  air  within  this  chamber  is  placed  under  a  higher 
pressure  than  the  atmospheric.  Following  the  channel  of 
least  resistance,  the  air  then  escapes  through  the  trachea  to 
the  outside.  The  opposite  movement  causes  the  outside 
air  to  flow  into  the  bellows,  because  the  pressure  therein  is 
then  lower  than  the  atmospheric.  It  is  to  be  noted  especially 
that  the  lung  is  a  perfectly  flaccid  organ  and  cannot  vary  its 
capacity  by  active  means.  This  statement  leads  us  to  infer 
that  the  changes  in  its  size  are  to  be  referred  to  an  outside 
factor,  resident  in  the  wall  of  the  chest.  It  is  to  be  observed 
that  the  surface  of  the  lung  is  everywhere  in  firm  contact 
with  the  thoracic  wall  and  must,  therefore,  pursue  the  same 
course  as  the  latter.  The  primary  factor  is  the  wall  of  the 
chest,  the  position  of  which  is  changed  by  muscular  activity. 
Its  outward  movement  causes  the  lungs  to  be  expanded,  while 
its  inward  movement  causes  them  to  be  compressed.  During 
the  former  phase  a  certain  amount  of  air  is  allowed  to  flow 
into  the  lungs.  It  is  again  expelled  during  the  subsequent 
phase  of  compression  of  these  organs. 

The  Gills. — Very  similar  conditions  are  met  with  in  the 
aquatic  animals.  The  place  of  the  lung  is  here  taken  by  the 
gill-plates,  representing  several  plate-like  prolongations  of 
tissue  containing  an  intricate  network  of  capillaries.  They 
are  limited  externally  by  a  layer  of  epithelium.  As  the 
water  rushes  across  their  surfaces  a  part  of  its  free  oxygen  is 
transferred  to  the  blood.  The  latter  in  turn  gives  off  carbon 


186  RESPIRATION 

dioxid  to  the  water.  This  oxygen  is  held  in  a  free  state  in 
the  water  and  is  not  derived  from  the  watery  molecule  itself. 
Accordingly,  fish  are  able  to  exist  in  an  aquarium  only 
in  the  presence  of  free  oxygen.  A  very  simple  way  of  keep- 
ing the  water  well  supplied  with  this  gas  is  to  allow  certain 
plants  to  grow  therein,  because  plants  liberate  oxygen  under 
the  influence  of  sunlight.  It  is  to  be  noted,  however,  that 
this  oxygen  is  of  metabolic  origin,  i.e.,  it  is  liberated  in  the 
course  of  the  assimilative  processes  within  the  green  parts 
of  the  plant,  and  is  not  expired  by  them. 

This  brings  to  our  mind  a  common  misconception.  The 
idea  has  become  prevalent  that  plants  inhale  carbon  dioxid 
and  exhale  oxygen,  thereby  tending  to  maintain  the  composi- 
tion of  the  atmospheric  air  for  the  benefit  of  animal  life. 
While  it  is  quite  true  that  the  existence  of  the  animals  is 
closely  dependent  upon  that  of  the  plants,  it  is  to  be  noted 
that  the  consumption  of  carbon  dioxid  and  liberation  of 
oxygen  by  the  latter  is  distinctly  a  matter  of  metabolism. 
Plants  as  well  as  animals  inhale  oxygen  and  exhale  carbon 
dioxid,  but  the  former  require  in  addition  carbon  dioxid  in 
order  to  be  able  to  build  up  their  substance.  In  the  course 
of  this  process  of  assimilation  a  certain  amount  of  super- 
fluous oxygen  is  liberated  which  is  transferred  to  the  medium 
in  which  they  are  growing.  This  explains  the  very  low 
percentage  of  carbon  diexid.  in  densely  wooded  regions. 

The  Respiratory  Cycle. — The  phase  of  expansion  of  the 
lung  is  designated  as  inspiration,  and  its  compression  as 
expiration.  Both  periods  together  form  the  respiratory 
cycle.  A  distinct  pause  is  not  present,  the  chest  being  held 
either  in  the  inspiratory  or  expiratory  position.  The  number 
of  the  respiratory  cycles  varies  in  accordance  with  the  age, 
sex,  size  and  activity  of  the  animal.  Other  conditions, 
such  as  the  temperature  of  the  medium,  barometric  pressure; 
and  season  of  the  year  also  exert  a  distinct  influence  upon  the, 
rate  of  respiration.  The  adult  human  male  respires  about 
16  times  in  a  minute,  and  the  human  female  17  to  18  times. 
Muscular  exercise  increases  the  respiratory  frequency  con- 
siderably, because  a  larger  amount  of  oxygen  is  then  needed 
to  satisfy  the  metabolic  requirements  of  the  body,  while  a 


THE    ELEMENTARY    LUNG  187 

larger  amount  of  carbon  dioxid  is  liberated.  Small  animals 
respire  more  frequently  than  larger  ones,  because  they  suffer 
a  greater  loss  of  heat  in  comparison  with  the  mass  of  their 
body.  A  loss  of  heat  can  only  be  compensated  for  by 
a  correspondingly  greater  production  of  heat,  i.e.,  by  activity 
and  more  intense  oxidations.  Increases  in  the  body-tem- 
perature are  usually  associated  with  a  greater  respiratory 
activity.  Lowering  the  outside  temperature  produces  a 
similar  result,  because  the  greater  loss  of  heat  occasioned 
thereby  must  be  balanced  by  intensifying  the  metabolism 
of  the  tissues.  For  this  reason,  the  mammals,  with  the 
exception  of  the  hibernating  animals,  are  more  active  during 
the  winter  months  and  consume  at  this  time  much  larger 
amounts  of  food. 


CHAPTER  XVIII 
THE   MECHANICS    OF   RESPIRATION 

The  Larynx  and  Trachea. — The  respiratory  mechanism 
of  the  mammals  consists  of  the  lungs,  one  on  each  side  of  the 
body,  and  a  passage  by  means  of  which  communication  is 
established  with  the  outside  air.  Entrance  to  this  passage 
is  gained  either  through  the  nose  or  mouth.  These 
chambers  open  posteriorly  into  a  common  space,  the  pharynx, 
whence  the  windpipe  or  trachea  leads  downward  through  the 


FIG.  81. — Diagram  of  an  elementary  lung.  S,  stigma;  O,  oxygen 
diffusing  from  air  of  saccule  into  tissue  fluids;  CO*,  diffusing  in  reverse 
direction. 

anterior  region  of  the  neck.  Opposite  the  second  pair  of 
ribs  it  divides  into  two  tubes,  known  respectively  as  the  right 
and  left  bronchus.  The  pharyngeal  cavity  is  continued 
onward  as  the  gullet  or  esophagus,  a  membranous  tube 
which  lies  directly  behind  the  trachea  and  traverses  the 
entire  length  of  the  chest  to  gain  access  to  the  stomach. 

The  uppermost  segment  of  the  trachea  is  modified  to  form 
a  special  organ  for  phonation,  which  is  known  as  the  larynx. 
Entrance  to  this  cavity  is  gained  through  an  aperture  which 

188 


THE    MECHANICS    OF    RESPIRATION  189 

is  guarded  by  a  leaf-like  lid,  called  the  epiglottis.  The  food 
entering  the  pharynx  is  projected  downward  into  the  esopha- 
gus and  cannot  escape  into  the  respiratory  passage,  because 
the  orifice  leading  into  the  larynx  is  at  this  time  at  least  par- 
tially closed.  It  should  be  noted,  however,  that  the  epiglottis 
is  not  the  only  factor  concerned  in  this  closure,  because  the 
entrance  of  food  into  the  trachea  is  already  guarded  against 
by  the  backward  movement  of  the  tongue  and  the  elevation 
of  the  larynx.  Nevertheless,  it  cannot  be  denied  that  the 
epiglottis  serves  as  a  special  means  of  shutting  out  fluids 
and  liquefied  foods. 

It  is  also  to  be  observed  that  the  respiratory  movements 
are  inhibited  during  the  act  of  swallowing,  so  that  the  food 
cannot  be  drawn  into  the  larynx.  When,  however,  the  acts 
of  swallowing  and  inspiration  are  not  properly  correlated, 
small  particles  of  food  and  liquids  may  be  diverted  into  the 
respiratory  channel  and  give  rise  to  an  intense  irritation  of 
its  lining  membrane.  This  excitation  induces  the  act  of 
coughing,  a  reflex  contraction  of  the  expiratory  muscles 
furnishing  a  powerful  blast  of  air  which  is  forced  through 
the  cavity  of  the  mouth.  Its  purpose  is  the  dislodgment  of 
the  irritating  particle.  The  same  principle  is  involved  in  the 
act  of  sneezing.  In  this  case,  however,  the  air  is  diverted 
through  the  nasal  cavity. 

A  short  distance  below  the  epiglottis  lie  the  vocal  cords, 
two  transverse  bands  of  fibrous  tissue  which  project  far  into 
the  lumen  of  the  laryngeal  cavity.  Their  position  and  struc- 
ture permits  them  to  vibrate  in  consequence  of  the  impacts 
produced  by  the  expiratory  blasts  of  air.  The  sounds  pro- 
duced by  these  membranous  bands  are  modified  by  the 
vibration  of  other  parts,  such  as  the  walls  of  the  chest  and 
larynx,  the  epiglottis,  pharynx,  and  membranous  structures 
situated  above  this  cavity.  It  will  be  seen,  therefore,  that 
the  phonating  organ  of  the  mammals  is  constructed  after 
the  principle  of  an  ordinary  blow-instrument,  such  as  a  trum- 
pet. The  essential  part  of  the  latter  is  a  tube,  the  orifice  of 
which  is  partially  closed  by  a  transverse  strip  of  thin  metal. 
By  blowing  forcibly  through  this  tube  the  aforesaid  band  is 
made  to  vibrate  and  to  produce  oscillations  of  a  similar 


190  RESPIRATION 

character  in  the  neighboring  air.  The  latter  are  conveyed 
into  consciousness  through  the  agency  of  the  ear. 

Sounds  of  high  and  low  pitch  are  occasioned  by  varying 
the  tenseness  of  the  vocal  cords,  because  when  thoroughly 
tightened  these  bands  cannot  vibrate  so  freely  as  when  re- 
laxed. Changes  of  this  character  follow  one  another  in 
rapid  succession  during  the  production  of  coordinate  sounds, 
such  as  are  used  in  speaking.  They  are  caused  by  finely 
adjusted  movements  of  the  cartilages  of  the  larynx  in 
consequence  of  the  contractions  of  special  sets  of  muscles. 

The  trachea  is  a  firm,  resistant  tube,  consisting  of  a  series 
of  rings  of  cartilage  which  are  joined  at  their  edges  by  mem- 
branous septa.  This  tube  measures  about  four  inches  and  a 
half  in  length  and  about  three-quarters  of  an  inch  in  diameter. 
The  first  ring  is  modified  to  aid  in  the  process  of  phonation 
and  forms,  therefore,  a  part  of  the  larynx.  It  is  known  as  the 
cricoid  cartilage.  Posteriorly,  where  the  trachea  lies  in  con- 
tact with  the  esophagus,  the  cartilaginous  rings  are  incom- 
plete and  flattened.  In  spite  of  this  structural  deficiency, 
however,  the  tube  as  a  whole  possesses  a  resistance  sufficient 
to  prevent  its  collapse  under  the  varying  degrees  of  pressure 
established  by  the  respiratory  movements. 

The  Bronchi  and  Lungs. — Opposite  the  fourth  dor  al 
vertebra  the  trachea  divides  into  two  smaller  tubes  which 
are  termed  the  bronchi.  Immediately  upon  their  entrance 
into  the  lung,  these  tubes  break  up  into  a  number  of  smaller 
ones,  which  are  designated  as  bronchioles.  The  smallest  of 
these  are  without  cartilaginous  support  and  terminate  in 
elongated  saccules,  called  infundibula.  Each  infundibulum 
is  divided  by  incomplete  partitions  into  a  number  of  smaller 
compartments  which  are  termed  air-cells  or  alveoli. 

Thus,  it  will  be  seen  that  the  mammalian  lung  possesses 
certain  structural  peculiarities  which  cause  it  to  resemble 
very  closely  the  lung  of  the  lowest  forms.  Inasmuch  as  the 
infundibulum  corresponds  really  to  the  greatly  dilated  end 
of  the  bronchiole,  it  may  justly  be  compared  to  the  pouch- 
like  lung  of  the  amphibians.  It  should  be  remembered, 
however,  that  the  long  diameter  of  these  air-spaces  measures 
less  than  1.0  mm.  (J^o  of  an  inch),  whereas  the  length  of  the 


THE    MECHANICS    OF    RESPIRATION  191 

expanded  lung  of  a  frog  of  medium  size  amounts  to  about 
4  cm.  (13^  inches).  The  alveoli  are  still  smaller  in  size, 
measuring  on  an  average  only  about  120/x  in  length,  but 
since  there  are  about  400  millions  of  them  in  each  lung,  the 
total  surface  presented  by  them  to  the  respiratory  air  is 
surprisingly  large.  Approximate  measurements  have  shown 
that  the  capillaries  of  the  lungs  possess  an  aggregate  sur- 
face of  about  125  sq.  m.  (300  square  feet).  Since  the  surface 


FIG.  82. — Diagram  illustrating  the  arrangement  of  the  infundibula. 
B,  bronchiole;  D,  infundibular  duct;  J,  infundibulum ;  A,  alveolus;  S, 
interinfundibular  space,  occupied  by  capillaries. 

of  the  body  of  a  man  of  medium  size  measures  only  about 
1.25  sq.  m.  it  will  be  seen  that  the  sheet  of  blood  exposed 
to  the  air  in  the  lungs  for  purposes  of  diffusion,  is  almost  100 
times  as  large  as  that  of  the  body-surface.  These  figures 
are  only  approximately  correct  and  are  given  here  merely  to 
impress  upon  the  reader  the  enormous  size  of  the  layer  of  blood 
.engaged  in  external  respiration. 

The  different  infundibula  are  bound  together  to  form 
lobules,  and  several  lobules  to  form  a  lobe.  The  right  lung 
consists  of  three  lobes,  and  the  left  one  of  two.  The  external 


192 


HESPIEATION 


surface  of  each  is  invested  by  a  serous  membrane,  known  as 
the  pleura.  This  lining  is  reflected  from  the  root  of  each 
organ  to  cover  the  entire  internal  surface  of  the  wall  of  the 
chest.  Thus,  the  pleura  really  consists  of  two  layers: 
namely,  one  upon  the  surface  of  the  lung  and  one  upon  the 
inner  aspect  of  the  thoracic  wall.  The  former  is  designated 


FIG.  83. — Human  respiratory  apparatus  showing  the  branching  of  the 
bronchi  in  the  interior  of  the  lung?.     (Duval.) 

as  visceral  pleura,  and  the  latter,  as  parietal  pleura.  Their 
opposing  surfaces  lie  in  firm  contact  with  one  another,  and 
are  moistened  with  a  small  quantity  of  a  serous  secretion, 
which  is  termed  the  pleural  fluid.  The  purpose  of  the  latter 
is  to  prevent  friction  while  the  lungs  change  their  size  and 
position  during  the  successive  respiratory  phases. 

The  Thorax. — The  cavity  of  the  thorax  is  a  subdivision  of 
the  cavity  of  the  trunk,  its  floor  being  formed  by  a  musculo- 
tendinous  septum  which  is  called  the  diaphragm.  This 


THE    MECHANICS    OF    RESPIRATION  193 

membrane  is  stretched  transversely  across  this  cavity  about 
opposite  the  lower  tip  of  the  sternum.  The  thorax  exhibits 
a  conical  outline,  its  tapering  upper  expanse  advancing  as  far 
as  the  base  of  the  neck.  Posteriorly,  it  is  limited  by  the  spinal 
column,  laterally  by  the  ribs,  and  in  front  by  the  sternum. 
The  lungs  occupy  this  entire  space  with  the  exception 
of  its  median  extent  which  gives  lodgment  to  the  heart  and 
large  bloodvessels,  and  is  designated  as  the  mediastinum. 

It  will  be  seen,  therefore,  that  the  lungs  are  situated  in  a 
closed  compartment,  and  communicate  with  the  outside 
only  through  a  relatively  narrow  passage,  the  trachea. 
Their  surfaces  lie  everywhere  in  intimate  contact  with  the 
inner  surface  of  the  thoracic  wall.  This  position  they  must 
retain,  because  the  pleural  space  is  closed  and  thoroughly 
protected  against  the  atmospheric  air  by  the  ribs  and 
adjoining  intercostal  muscles  and  membranes. 

A  separation  between  the  outer  surface  of  the  lung  and  the 
chest  wall  can  only  be  effected  by  the  establishment  of  a  free 
communication  between  the  pleural  cavity  and  the  outside. 
The  air  entering  through  this  opening  into  the  formerly 
potential  pleural  space,  permits  the  lung  tissue  to  draw  away 
from  the  wall  of  the  chest.  This  property  of  recoil  is  resident 
in  the  elastic  fibers  of  the  walls  of  the  alveoli.  Inasmuch  as 
every  air-sac  is  thereby  greatly  reduced  in  size,  the  volume 
and  capacity  of  the  entire  organ  must  also  be  considerably 
diminished.  This  phenomenon  is  designated  as  the  collapse 
of  the  lung.  When  in  this  state,  the  lung  can  no  longer  be 
expanded  in  the  normal  way,  because  it  now  lacks  the  force 
of  the  chest  wall  acting  upon  its  external  surface.  But, 
a  collapsed  organ  is  not  entirely  free  from  air,  because  the 
walls  of  the  membranous  bronchioles  are  brought  together 
before  the  air  has  had  sufficient  time  to  escape  from  the 
alveoli.  Consequently,  a  collapsed  lung  will  float,  and  is 
able  to  carry  a  considerable  weight  in  addition  to  its  own. 
The  collapse  of  one  lung  does  not  necessarily  prove  fatal, 
because  this  loss  is  compensated  for  by  a  more  complete 
expansion  of  the  opposite  organ. 

The  Expansion  of  the  Lung. — In  order  to  be  able  to  under- 
stand the  manner  in  which  the  lungs  are  expanded,  the 

13 


194  RESPIRATION 

following  points  already  alluded  to  above,  should  be  kept 
firmly  in  mind: 

(a)  The  surface  of  the  lung  is  kept  in  absolute  contact  with 
the  internal  surface  of  the  wall  of  the  chest. 

(6)  The  lung  is  a  perfectly  passive  organ  and  possesses  no 
means  by  which  it  could  increase  or  decrease  its  size  and 
capacity  in  an  active  way. 

(c)  The  active  factor  in  respiration  is  the  wall  of 
the  chest.  It  is  moved  outward  during  inspiration  by  the 
contraction  of  certain  muscles,  forming  the  group  of  the 


£  J 


FIG.  84. — Diagram  showing  the  position  of  the  diaphragm  and  adjoin- 
ing wall  of  the  trunk  on  inspiration  and  expiration.  E,  expiration;  J, 
inspiration.  The  diaphragm  moves  downward  and  the  walls  of  the 
trunk  outward,  increasing  the  size  of  the  complementary  space  C.  The 
slight  depression  at  H  is  caused  by  the  apical  portion  of  the  heart. 

inspiratory  muscles.  When  these  muscles  cease  to  contract, 
the  wall  of  the  chest  returns  into  its  original  position.  This 
movement  constitutes  the  expiratory  phase  of  the  respiratory 
cycle.  We  shall  see  later  that  inspiration  is  an  active  process 
throughout,  whereas  expiration  is  passive  under  ordinary 
circumstances,  i.e.,  it  is  accomplished  by  gravity  and  elastic 
recoil  and  not  by  the  contraction  of  a  special  set  of  muscles. 
Having  established  this  principle,  that  these  movements  of 
the  wall  of  the  chest  are  responsible  for  the  variations  in  the 
size  of  the  lungs,  let  us  briefly  analyze  the  manner  in  which 
the  capacity  of  the  thorax  is  altered.  In  the  first  place,  do 
not  expect  to  find  very  striking  fluctuations,  because  nature 


THE   MECHANICS   OF   RESPIRATION 


195 


always  works  efficiently  with  the  least  expenditure  of  energy. 
Thus,  while  it  is  apparent  in  most  animals  that  the  size  of 
the  chest  is  larger  during  inspiration  than  during  expiration, 
this  difference  is  not  considerable  under  ordinary  circum- 
stances. It  should  also  be  noted  that  the  chest  is  enlarged 
in  all  directions,  but  chiefly  in  three:  namely,  from  above 
downward,  from  before  backward,  and  from  side  to  side. 

The  increase  in  its  height  is  produced  by  the  contraction 
of  the  diaphragm.  This  membrane  consists  of  a  central 
tendinous  portion,  to  the  margin  of 
which  are  attached  the  different  mus- 
cle fibers.  The  latter  pursue  a  course 
radially  downward  and  are  finally  at- 
tached to  the  inner  surfaces  of  the 
lower  ribs.  Accordingly,  this  musculo- 
tendinous  septum  presents  a  curved 
outline,  its  convexity  being  turned 
into  the  cavity  of  the  chest.  It  will 
be  seen,  therefore,  that  the  contraction 
of  its  muscle  fibers  must  pull  its  ten- 
dinous portion  downward,  thereby 
increasing  the  vertical  diameter  of  the 
chest  at  the  expense  of  that  of  the 
abdominal  cavity. 

The  antero-posterior  and  transverse 
diameters  of  the  chest  are  increased  by  spinal  column 
the  upward  movement  of  the  sternum 
and  ribs.  The  latter  are  hinged  upon  the  vertebrae  and 
are  united  with  the  sternum  by  means  of  short  pieces  of 
cartilage.  However,  since  the  hinder  end  of  each  rib  articu- 
lates with  the  corresponding  vertebra  in  two  places,  its 
movements  cannot  take  place  in  a  straight  line  up  and  down, 
but  must  follow  the  oblique  axis  of  rotation  of  these  joints. 
For  this  reason,  the  upward  movement  of  each  rib  necessi- 
tates a  moderate  outward  rotation  of  its  bony  portion  at  the 
angle  and  a  slight  torsion  of  its  cartilaginous  sternal  end. 

At  the  end  of  expiration  the  ribs  are  directed  obliquely 
downward,  so  that  the  sternum  assumes  a  much  lower  level 
than  after  the  completion  of  inspiration.  Ordinarily,  the 


FIG.  85.—  The  position 
of  the  ribs  on  inspiration 
(red)  and  expiration 
(black).  S,  sternum;  C, 


196  RESPIRATION 

range  of  this  movement  is  sufficient  to  permit  the  rib  below 
to  assume  the  level  of  the  next  rib  above.  Now,  since  the 
horizontal  diameters  of  the  chest  increase  from  above  down- 
ward, this  elevation  of  the  sternum  must  bring  the  larger 
areal  expanse  of  the  chest  at  the  rib  below  to  that  of  the  next 
rib  above.  Moreover,  since  the  ribs  are  relatively  inflexible, 
their  upward  movement  must  push  the  sternum  forward, 
thereby  increasing  the  diameter  of  the  chest  from  before 
backward. 

The  Deflation  of  the  Lung. — The  three  factors  which 
should  be  held  responsible  for  the  decrease  in  the  size  of  the 
lung,  are  gravity,  elastic  recoil,  and  muscular  action.  As 
has  been  stated  above,  normal  expiration  is  very  largely  a 
passive  act  and  is  not  participated  in  by  any  muscles  with 
the  possible  exception  of  the  internal  inter costals.  However, 
when  the  gas  interchange  is  to  be  hastened,  other  muscles  are 
called  into  play  until  even  the  expiratory  movement  becomes 
distinctly  active.  Thus,  it  may  be  stated  that  the  hard  parts 
are  carried  into  their  former  position  by  their  own  weight 
as  soon  as  the  inspiratory  muscles  have  ceased  to  act  upon 
them.  Besides,  the  cessation  of  the  inspiratory  effort  per- 
mits the  parts  previously  put  on  the  stretch  to  recoil  until 
they  have  assumed  their  former  positions.  Lastly,  certain 
muscles  may  be  activated  during  the  expiratory  phase  in  an 
endeavor  to  hasten  the  inward  movement  of  the  chest  wall. 

This  action  is  well  illustrated  by  the  movements  of  the 
diaphragm.  When  this  muscular  septum  contracts  during 
inspiration,  it  is  flattened  and  encroaches  upon  the  space  of 
the  abdomen.  The  abdominal  organs  are  thereby  put  under 
a  certain  pressure,  while  the  lung  tissue  is  pulled  upon  and 
expanded.  Now,  since,  the  abdominal  cavity  is  limited 
posteriorly  by  the  unyielding  vertebral  column  and  below 
by  the  pelvis,  an  interchange  of  pressure  can  only  be  accom- 
plished through  its  soft  anterior  and  lateral  walls.  Accord- 
ingly, we  find  that  the  downward  progression  of  the 
diaphragm  and  neighboring  liver  and  stomach  causes  the 
abdominal  wall  to  be  pushed  outward.  The  connective 
tissue  and  muscles  of  the  latter  are  thereby  put  on  the  stretch. 
Immediately  upon  the  completion  of  the  contraction  of  the 


THE   MECHANICS   OF   RESPIRATION 


197 


diaphragm,  these  parts  recoil  and  force  the  abdominal  viscera 
back  into  the  space  vacated  by  the  lung.  Hence,  the  upward 
movement  of  the  diaphragm,  is  accomplished  by  two  factors : 
namely,  the  pull  exerted  upon  its  upper  surface  by  the  recoil- 
ing lung  and  the  push  imparted  upon  its  under  surface  by 
the  recoiling  abdominal  wall. 


FIG.  86. — Diagrammatic  sections  of  the  body  in  A,  inspiration;  and  B, 
expiration;  tr,  trachea;  st,  sternum;  D,  diaphragm;  ab,  abdominal  walls. 
The  shading  roughly  indicates  the  stationary  air.  (From  Huxley s  "Lessons 
in  Elementary  Physiology,"  Macmillan  Co.,  Publishers.) 

Diaphragmatic  and  Costal  Breathing. — Under  ordinary 
circumstances  the  descent  of  the  diaphragm  is  sufficient  to 
yield  an  adequate  interchange  of  the  gases,  but  when  a  more 
ample  ventilation  of  the  lungs  is  to  be  effected,  the  action  of 
this  muscle  must  be  augmented  by  that  of  other  respiratory 
muscles.  The  normally  diaphragmatic  type  of  respiration 
is  then  changed  into  one  possessing  a  distinct  costal  character. 
Naturally,  this  change  necessitates  the  participation  of  the 
upper  ribs  in  respiration.  Costal  respiration  is  usually  said 
to  be  characteristic  of  woman,  and  diaphragmatic  respiration 
of  men,  but  this  distinction  is  not  based  upon  physiological 


198  RESPIRATION 

differences,  because  those  women  who  are  not  in  the  habit 
of  wearing  corsets,  likewise  respire  chiefly  with  the  aid  of  the 
diaphragm.  In  accordance  with  the  preceding  discussion, 
it  may  be  concluded  that  costal  respiration  must  be  resorted 
to  whenever  the  descent  of  the  diaphragm  is  hindered. 
Such  an  impairment  may  easily  be  brought  about  by  sitting 
in  a  cramped  position  in  a  chair,  and  especially  when  the 
stomach  is  well  filled  with  food. 

The  successive  involvement  of  the  different  muscles  of 
respiration  may  easily  be  observed  in  a  person  indulging  in 
physical  exercise.  To  begin  with,  solely  the  diaphragm  and 
lower  intercostal  muscles  are  at  work,  while  later  on  the 
gradually  increasing  metabolism  necessitates  the  simultane- 
ous activation  of  the  upper  intercostals  and  accessory  muscles 
of  respiration.  No  doubt,  everyone  of  us  is  familiar  with  the 
picture  presented  by  a  person  who  has  just  completed  a  long- 
distance race.  We  are  impressed  by  the  set  character  of  his 
face,  his  opened  angular  mouth,  the  cord-like  prominence  of 
the  sterno-cleido-mastoid  muscles,  and  the  forward  bend  of 
his  neck  and  shoulders.  All  these  muscular  actions  favor 
the  fixation  of  the  upper  ribs,  so  that  it  becomes  possible  to 
act  upon  the  others  with  much  greater  force.  Similar  fixed 
points  are  established  at  this  time  below,  but  principally  by 
the  quadrati  lumborum  and  allied  muscles  which  give  support 
to  the  lower  ribs. 

Normal  and  Forced  Breathing. — In  accordance  with  the 
preceding  discussion,  it  may  be  concluded  that  the  respiratory 
movements  may  present  either  a  normal  or  &  forced  character. 
As  long  as  the  metabolism  retains  a  moderate  intensity,  the 
diaphragm  with  the  possible  addition  of  the  lower  intercostals, 
suffices  to  give  an  adequate  ventilation  of  the  lungs  and 
interchange  of  the  gases.  Contrariwise,  a  constantly  in- 
creasing number  of  respiratory  muscles  must  be  brought  into 
play  when  the  metabolism  is  augmented.  The  forced  in- 
spiratory  movements  are  then  associated  with  forced  ex- 
pirations, necessitating  the  participation  of  the  different 
muscles  of  expiration. 

The  action  of  the  diaphragm  is  augmented  first  of  all  by  the 
intercostal  muscles,  consisting  of  two  layers  of  fibers  which  pass 


THE    MECHANICS    OF    RESPIRATION 


199 


between  the  successive  ribs  in  opposite  directions  to  one 
another.  Those  of  the  external  layer  pursue  a  course 
obliquely  downward  and  forward  to  the  rib  below,  while 
those  of  the  internal  layer  run  obliquely  upward  and  forward 
to  the  rib  above.  Their  contraction  keeps  the  intercostal 
spaces  in  a  firm  condition,  so  that  these  relatively  soft 
areas  of  the  chest  wall  cannot  be  drawn  inward  by  the  recoil- 


S    J 


B 


FIG.  87. — Diagram  illustrating  the  action  of  the  intercostal  muscles. 
S,  sternum;  V,  vertebrae;  A  and  B,  two  consecutive  ribs;  EE\,  external 
intercostal  muscle;  II\,  internal  intercostal  muscle.  The  contraction 
of  the  first  raises  the  ribs  (//) ,  while  the  contraction  of  the  second  lowers 
them  (III).  The  distance  SV  is  now  shortened. 

ing  lung  tissue.  Secondly,  they  exert  a  very  characteristic 
action  upon  the  position  of  the  ribs,  because  the  external  fibers 
move  them  upward  while  the  internal  ones  depress  them.  In 
explaining  these  movements  it  should  be  remembered  that 
the  contraction  of  a  muscle  brings  its  point  of  insertion 
closer  to  its  point  of  attachment.  Now,  since  the  points 
of  attachment  of  the  external  fibers  lie  nearer  the  vertebral 
column  than  their  points  of  insertion  upon  the  rib  below,  their 
contraction  must  move  the  sternum  upward.  Likewise, 
since  the  points  of  insertion  of  the  internal  fibers  upon  the  rib 
above  are  situated  farther  away  from  the  spinal  column  than 
their  points  of  attachment,  their  contraction  must  pull  the 
sternum  downward.  The  raising  of  the  ribs  is  aided  in 
all  probability  by  other  muscles,  but  principally  by  the  leva- 
tores  costarum. 


CHAPTER  XIX 
THE  CHEMISTRY  OF  RESPIRATION 

The  Effect  of  the  Respiratory  Movements  upon  the  Air 

Content  of  the  Lungs. — It  need 
scarcely  be  emphasized  that  the 
inspiratory  expansion  of  the 
lungs  produces  an  area  of  low 
pressure  within  the  pulmonary 
passages,  whereas  the  expiratory 
movement  places  the  air  in  these 
channels  under  a  higher  pressure 
than  the  atmospheric.  In  con- 
sequence of  these  changes  in 
pressure  a  certain  amount  of  air 
is  made  to  flow  into  the  lungs 
with  each  inspiratory  phase. 
This  air  is  again  expelled  during 
the  succeeding  expiratory  period. 
It  will  be  noted,  however,  that 
the  quantity  of  air  actually 
moved  during  each  respiratory 
cycle  is  small  in  comparison  with 
the  total  amount  of  air  present 
in  the  lungs. 

This  point  may  easily  be 
proved  by  measuring  the  volume 
of  the  respiratory  air  by  means 
of  an  instrument  which  is- 
modelled  after  an  ordinary  gas- 
ometer and  is  known  as  a 
spirometer  (Fig.  88).  This  in- 
strument consists  of  a  cylinder 
(B)  filled  with  water,  in  which 
200 


FIG.  88. — Wintrich's  modifi- 
cation of  Hutchinson's  spirom- 
eter. (R  etcher  t.) 


THE    CHEMISTRY    OF    RESPIRATION 


201 


is  suspended  a  smaller  cylinder  (A)  containing  air.  The 
weight  of  the  latter  is  accurately  counterpoised  (G),  so  that 
it  moves  with  the  least  possible  resistance.  The  tube  (C) 
enters  through  the  outside  cylinder  and  is  continued  upward 
to  the  level  of  the  water  in  the  inside  compartment.  When 
air  is  expired  through  this  tube,  cylinder  A  rises,  its  excursion 
being  registered  by  a  pointer  resting 
upon  a  scale. 

By  means  of  the  spirometer  it  has 
been  ascertained  that  a  normal  adult 
inspires  each  time  only  about  500  c.c. 
pf  air,  which  is  designated  as  the  tidal 
Under  ordinary  circumstances 


vc 


St 


air. 

this  amount  is  added  to  about  3000  c.c. 
of  air  which  is  always  retained  in  the 
lungs.  The  latter  is  called  the  station- 
ary air.  If,  however,  a  forced  expira- 
tion is  resorted  to,  the  person  is  able 
to  exhale  1500  c.c.  in  addition  to  the 
tidal  air.  This  extra  quantity  is 
termed  the  supplemental  air.  Thus, 
the  stationary  air  is  really  made  up  of 
1500  c.c.  of  supplemental  air  and  1500 
c.c.  of  residual  air,  the  latter  consti- 
tuting that  portion  of  it  which  cannot 
be  expelled  even  by  most  powerful  air  respired.  T,  tidal 
expiration.  It  is  also  possible  to  inspire  ^jZ^KIl: 

Very  deeply,  and  to  take  in  1500  C.C.  in     residual     air;      St,    sta- 
tionary   air;     VC,    vital 


FIG.  89.  —  Volumes  of 


capacity. 


addition  to  the  tidal  air.  This  extra 
amount  is  called  the  complemental  air. 
Consequently,  an  adult  person  of  medium  size  is  able  to 
accommodate  about  5000  c.c.  of  air  in  his  lungs.  This 
amount  constitutes  the  total  lung  capacity.  The  sum  of  the 
supplemental,  tidal  and  complemental  air  amounts  to  3500 
c.c.  It  reveals  the  vital  capacity  of  the  lungs,  a  factor  fre- 
quently made  use  of  in  ascertaining  the  functional  power  of 
the  thorax. 

The  Changes  in  the  Respired  Air. — When  a  comparison  is 
made  between  samples  of  expired  and  inspired  air,  it  will 


202  RESPIRATION 

be  noted  that  they  differ  from  one  another  in  a  chemical  as 
well  as  physical  way.  Ordinary  atmospheric  air  con- 
tains in  100  volumes:  20.96  per  cent,  of  oxygen,  79.00  per 
cent,  of  nitrogen,  and  0.04  per  cent,  of  carbon  dioxid.  Ex- 
pired air,  on  the  other  hand,  embraces  only  16.50  per  cent,  of 
oxygen,  79.50  per  cent,  of  nitrogen,  and  4.0  per  cent,  of  carbon 
dioxid.  Accordingly,  it  will  be  seen  that  the  inspired  air 
loses  about  4.0  per  cent,  of  oxygen,  and  gains  a  corresponding 
amount  of  carbon  dioxid.  The  slight  gain  in  nitrogen  is  due 
to  the  fact  that  the  expired  air  is  always  contaminated  with 
mucous  and  cellular  particles  which  have  been  torn  loose 
from  the  respiratory  passage. 

Among  the  physical  differences  should  be  mentioned  the 
fact  that  the  expired  air  is  always  warmer  than  the  inspired, 
but  whether  the  former  is  actually  heated  to  the  temperature 
of  the  body  depends  upon  the  initial  temperature  of  the  air 
taken  in  as  well  as  upon  the  length  of  time  during  which  it  is 
retained  in  the  lungs.  Secondly,  the  expired  air  invariably 
contains  a  larger  amount  of  water  vapor,  which  is  derived 
chiefly  from  the  lining  of  the  outer  respiratory  passage. 
The  quantity  of  water  which  may  be  lost  by  the  body  in 
this  way,  often  amounts  to  300  c.c.  in  the  course  of  twenty- 
four  hours.  Thirdly,  the  volume  of  the  expired  air  is  some- 
what smaller  than  that  of  the  inspired  (/^o)>  because  a 
moderate  portion  of  the  oxygen  taken  in  unites  with  hydro- 
gen to  form  water.  Accordingly,  not  all  the  oxygen  inhaled 
combines  with  carbon  to  form  carbon  dioxid,  otherwise 
the  amounts  of  these  gases  would  be  accurately  balanced. 
Lastly,  the  expired  air  embraces  certain  organic  admixtures, 
such  as  fragments  of  the  lining  of  the  respiratory  passage, 
and  mucus.  While  these  constituents  form  a  distinctly 
injurious  element  in  expired  air,  they  should  not  be  held 
responsible  for  the  repulsive  odor  sometimes  emitted  by  the 
breath.  Such  odors  usually  find  their  origin  in  decaying 
particles  of  food  and  in  the  putrefying  emissions  of  chronically 
inflamed  areas  of  the  nasal  and  pharyngeal  cavities. 

Attention  should  also  be  called  at  this  time  to  the  fact  that 
while  only  a  comparatively  small  quantity  of  air  is  moved 
with  each  respiratory  movement,  the  total  amount  respired 


THE    CHEMISTRY    OF    RESPIRATION 


203 


in  the  course  of  twenty-four  hours  equals  about  10,000  liters, 
or  350  to  400  cubic  feet.  From  this  astonishingly  large 
volume  of  air  the  body  abstracts  its  oxygen.  When  cal- 
culated at  4  per  cent.,  the  latter  equals  about  500  liters,  or 
18  cubic  feet.  It  need  scarcely  be  em- 
phasized that  these  figures  are  greatly 
increased  during  muscular  exercise. 

The  Interchange  of  the  Respiratory 
Gases.  —  It  has  been  noted  above  that 
the  lungs  are  only  partially  deflated  dur- 
ing expiration  and  retain  about  3000  c.c. 
of  stationary  air  after  each  ordinary  ex- 
piration. How  then  is  the  tidal  air  able 
to  reach  the  blood,  when  this  amount  of 
air  is  barely  large  enough  to  fill  the  outer 
respiratory  passage?  This  question  may 
be  answered  satisfactorily  by  simply  ap- 
plying the  principles  of  the  laws  of  diffu- 
sion to  the  mammalian  lung. 

The  tidal  air  in  the  outer  respiratory 
channels  possesses  practically  the  same 
composition  as  the  atmospheric  air. 
Consequently,  its  molecules  of  oxygen 
must  be  held  under  a  partial  pressure  of 
152  mm.  Hg,  while  those  of  carbon  dioxid 
must  be  under  a  partial  pressure  only 
slightly  above  zero.  In  the  blood,  on 

,r          /i          i          i      j.i  j-    i  f     sion  of  the  gases  be- 

the  other  hand,  the  partial  pressure  of  tween  the  tidal  air 
the  oxygen  must  be  much  lower,  namely,  and  the  blood.  T, 
something  like  100  mm.  Hg,  whereas  that  *?«*ea;  TA>  tidal 

j.      .j  *  '  ,        air;    B,    bronchi;    J, 

of  carbon  dioxid  must  be  high,  approach-  infundibuium;  c, 
ing  the  value  of  35  mm.  Hg.  Hence,  the  capillaries;  o,  oxygen 
molecules  of  oxygen  must  flow  in  a 
steady  stream  from  the  tidal  air  into 
the  blood,  where  they  combine  with  the 
hemoglobin  of  the  red  cells  to  form  oxy  hemoglobin.  Con- 
trariwise, the  molecules  of  carbon  dioxid  must  leave  the 
blood  and  pass  into  the  tidal  air.  These  diffusion  streams 
continue  as  long  as  the  differences  in  the  partial  pressures 


FIG.  90. — Diagram 
illustrating  the  diffu- 


dioxid, 


204  RESPIRATION 

of  these  gases  are  kept  up  by  the  successive  inspiratory 
renewals  of  the  air  in  the  outer  respiratory  passage.  Nitro- 
gen is  an  inert  gas,  and  merely  serves  as  the  medium  in 
which  this  diffusion  is  accomplished. 

Eupnoea,  Apnoea,  Dyspnoea  and  Asphyxia. — Whenever 
normal  amounts  of  oxygen  are  taken  in  and  normal  amounts 
of  carbon  dioxid  given  off,  the  body  is  said  to  be  in  equili- 
brium as  far  as  these  gases  are  concerned.  The  respiratory 
movements  then  possess  a  normal  frequency  and  amplitude. 
At  this  time  the  body  is  in  the  condition  of  eupncea. 

The  condition  of  apncea  signifies  a  superfluity  of  oxygen. 
Whenever  the  system  contains  too  large  an  amount  of  this 
gas,  the  respiratory  movements  are  discontinued  until  it  has 
again  been  reduced  to  its  normal  value.  Apncea,  therefore, 
is  characterized  by  a  cessation  of  the  respiratory  movements. 
It  is  a  matter  of  common  experience  that  this  condition  may 
be  produced  very  easily  by  respiring  three  or  four  times  in 
quick  succession:  The  breath  is  then  held  until  the  excess 
oxygen  has  been  used  up.  Apncea  may  also  be  induced  by 
inhaling  air  containing  a  larger  amount  of  oxygen  than 
usual.  The  partial  pressure  of  this  gas  is  thereby  increased, 
causing  its  molecules  to  enter  the  system  at  a  much  faster 
rate  than  normal. 

The  condition  of  dyspnoea  signifies  a  scarcity  of  oxygen, 
and  superfluity  of  carbon  dioxid.  It  is  characterized  by  an 
increased  respiratory  rate  and  force.  Dyspnoea  may  be 
produced  by  mechanical  as  well  as  chemical  means.  Thus, 
it  is  easily  conceivable  that  a  poorly  ventilated  state  of  the 
alveolar  contents  and  diminished  intensity  of  diffusion  must 
follow  the  partial  occlusion  of  the  trachea  by  foreign  bodies 
and  tumors.  The  same  result  may  be  obtained  by  the  inhala- 
tion of  an  inert  gas.  Carbon  monoxid  belongs  in  this  group 
of  agents,  because  it  prevents  the  ingo  of  oxygen  by  uniting 
with  the  hemoglobin  of  the  red  cells  to  form  carbon-monoxid- 
hemoglobin.  A  corpuscle  so  altered  loses  its  properties  as 
a  carrier  of  oxygen,  because  the  aforesaid  combination  of 
hemoglobin  and  carbon  monoxid  cannot  easily  be  destroyed. 

During  the  later  stage  of  dyspnoea  convulsive  muscular 
movements  set  in  which  are  associated  with  intense  inspira- 


THE    CHEMISTRY    OF    RESPIRATION  205 

tory  efforts.  After  the  oxygen  supply  has  been  completely 
exhausted,  these  spastic  inspirations  recur  at  long  intervals 
and  finally  cease  altogether.  The  heart  stops  beating  very 
soon  afterward,  its  right  side  as  well  as  the  central  veins 
then  being  greatly  distended  with  very  dark  blood.  This 
advanced  stage  of  dyspnoea  is  designated  as  asphyxia.  It 
indicates  a  deprivation  of  oxygen  and  an  excessive  accumula- 
tion of  carbon  dioxid. 

The  foregoing  discussion  should  allow  us  to  form  an  opinion 
regarding  the  manner  in  which  such  diseases  as  pneumonia 
terminate  life.  Pneumonia  is  an  inflammatory  reaction  of 
the  substance  of  the  lung,  as  against  pleurisy  which  is  an 
inflammation  of  its  outer  lining  membrane.  In  the  course 
of  this  disease  the  lining  cells  of  the  alveoli  give  off  a  certain 
amount  of  exudate  which  accumulates  in  the  infundibular 
spaces  until  the  latter  have  been  completely  blocked.  Obvi- 
ously, this  infiltration  must  render  these  spaces  functionally 
useless,  because  the  ordinary  partial  pressure  of  the  oxygen 
then  no  longer  suffices  to  drive  this  gas  through  the  exudated 
material.  Dyspnoea  sets  in  as  soon  as  a  sufficient  number  of 
alveoli  have  thus  been  rendered  functionally  useless.  Under 
favorable  circumstances,  this  stage  during  which  parts  of  the 
lungs  are  solidified,  is  followed  by  the  stage  of  resolution. 
The  exudated  material  is  again  absorbed,  thereby  permitting 
the  molecules  of  the  gases  to  traverse  the  alveoli  in  normal 
numbers. 

Ventilation. — The  object  of  ventilation  is  to  make  indoor 
conditions  suitable  for  indoor  life.  Consequently,  the  prob- 
lem involved  in  ventilation  is  essentially  a  physiological  one 
and  must  have  as  its  primary  object  the  chemical  composition 
of  the  air  breathed.  The  temperature  and  content  in  water 
vapors  of  the  latter  are  really  of  secondary  importance, 
although  as  high  a  percentage  as  0.3  to  1.0  of  carbon  dioxid 
may  readily  be  endured,  if  the  air  is  cool  and  relatively  dry. 
We  have  previously  noted  that  an  adult  person  inhales  about 
500  c.c.  of  air  seventeen  times  in  a  minute,  and  that  his 
output  of  carbon  dioxid  at  rest  amounts  to  17  liters  or  0.68 
cubic  foot  in  an  hour.  If  the  normal  amount  of  this  gas  is 
0.03  per  cent.,  its  percentage  in  28,000  liters  or  1000  cubic 


206  BESPIRATION 

feet  will  be  increased  to  about  0.1  per  cent,  in  the  course  of 
one  hour.  Consequently,  the  amount  of  fresh  air  required 
per  hour  to  keep  the  carbon  dioxid  at  0.06  per  cent.,  is 
0.03:0.68:100:z,  or  x  =  2000  cubic  feet.  In  this  calcula- 
tion an  allowance  should  be  made  for  the  size  of  the  person  and 
the  kind  of  work  performed  by  him. 

It  is  also  of  interest  to  note  that  rather  high  percentages 
of  carbon  dioxid  may  be  endured  without  great  discomfort, 
provided  the  air  is  cool  and  relatively  dry.  Optimum  condi- 
tions prevail  when  its  carbon  dioxid  content  does  not  exceed 
0.06  per  cent,  and  when  its  temperature  is  between  65  and 
68°  F.  with  a  relative  humidity  of  50  to  75  per  cent.  Cold 
stimulates  the  cutaneous  sense-organs,  and  augments  the 
tonus  of  the  bloodvessels  as  well  as  that  of  the  heart  muscle. 
The  circulation  is  intensified.  A  warm  and  stuffy  atmos- 
phere, on  the  other  hand,  relaxes  and  gives  rise  to  symptoms 
of  fatigue  and  exhaustion  in  spite  of  the  fact  that  its  content 
in  carbon  dioxid  may  be  low. 

It  need  scarcely  be  emphasized  that  air  containing  a  large 
amount  of  water  vapor,  prevents  the  moisture  upon  the 
surface  of  the  skin  from  evaporating,  and  hinders  thereby 
the  body  in  discharging  its  heat  in  this  manner.  Likewise, 
a  high  temperature  of  the  surrounding  air  diminishes  the 
loss  of  heat  from  the  lining  of  the  pulmonary  passage.  Both 
factors  combined  greatly  favor  heat-retention  and  eventually 
produce  those  uncomfortable  sensations  which  practically 
everybody  experiences  under  these  circumstances.  The 
production  of  currents  in  the  air  by  electric  fans  and  other 
means  diminishes  this  discomfort,  because  it  increases  heat- 
dissipation  by  constantly  bringing  fresh  air  in  contact  with 
the  surface  of  the  body. 

The  Respiratory  Interchange  at  High  Altitudes. — As  we 
ascend  to  a  higher  altitude,  the  barometric  pressure  gradually 
decreases,  reaching  a  value  of  about  one-half  the  coast 
standard  at  a  height  of  15,000  feet.  In  accordance  with  the 
laws  of  diffusion,  this  decline  in  the  atmospheric  pressure 
and  partial  pressures  of  the  constituents  of  the  air  must 
impair  the  influx  of  the  oxygen,  because  the  molecules  of  this 
gas  are  now  under  a  driving  power  considerably  less  than 


THE    CHEMISTRY    OF    RESPIRATION  207 

152  mm.  Hg.  Clearly,  the  fact  that  the  hemoglobin  of  the 
red  cells  possesses  a  natural  affinity  for  this  gas,  cannot  alter 
this  result,  because  the  readiness  with  which  these  elements 
combine  is  not  augmented  at  higher  levels  in  a  measure  to 
counteract  the  effect  of  the  loss  in  partial  pressure. 

This  difficulty  is  overcome  in  a  large  measure  by  the 
formation  of  many  new  red  corpuscles.  Thus,  it  appears 
that  the  scarcity  of  oxygen  arising  at  high  altitudes  stimulates 
the  corpuscle-producing  organs  to  greater  activity,  thereby 
placing  the  blood  in  possession  of  a  greater  number  of  oxygen 
carriers.  When  this  physiological  reaction  is  at  its  height, 
the  red-cell-count  may  be  something  like  8,000,000  per  cubic 
millimeter  of  blood,  as  against  the  normal  of  5,000,000  per 
cubic  millimeter.  Obviously,  this  change  must  influence  the 
interchange  of  the  oxygen  in  a  very  beneficial  manner, 
because  while  each  corpuscle  now  contains  a  somewhat  smaller 
amount  of  this  gas  than  at  a  lower  altitude,  the  total  amount 
of  the  latter  cannot  be  materially  diminished  in  view  of  the 
fact  that  the  number  of  the  carriers  'has  been  considerably 
augmented.  At  still  higher  levels,  however,  an  adequate 
interchange  of  oxygen  cannot  be  effected  even  with  the  aid  of 
this  physiological  reaction,  because  the  partial  pressure  of 
this  gas  is  then  so  greatly  reduced  that  the  aforesaid  com- 
pensation becomes  entirely  ineffective.  The  complex  of 
symptoms  frequently  experienced  at  high  altitudes,  con- 
stitutes the  so-called  mountain  sickness.  Heights  of  5000  to 
6000  m.  cannot  well  be  endured  by  most  people.  Still  greater 
heights  can  only  be  reached  with  the  aid  of  pure  oxygen. 

It  should  be  borne  in  mind,  however,  that  the  breathing 
of  pure  oxygen  by  a  healthy  person  is  not  without  its  dangers, 
because  excessive  amounts  of  this  gas  possess  a  poisonous 
action  upon  the  cells  of  the  tissues.  This  statement  implies 
that  the  activity  of  normal  living  matter  cannot  be  aug- 
mented by  " forced  draft,"  as  a  fire  might  be.  Oxygen  serves 
as  a  potent  remedial  agent  only  when  the  quality  of  the  blood 
is  to  be  restored  within  a  comparatively  brief  period  of  time. 
For  this  reason,  pure  oxygen  is  frequently  administered  in 
pneumonia  and  other  respiratory  diseases  when,  owing  to  the 
poor  ventilation  of  the  alveoli,  its  percentage  in  the  blood 


208  RESPIRATION 

has  been  greatly  diminished.  A  similar  reason  may  be  given 
for  the  administration  of  oxygen  during  ether  narcosis. 
It  mitigates  the  after-effects  of  this  narcotic  agent  in  an 
unmistakable  manner.  When  carefully  administered,  oxy- 
gen also  increases  the  power  of  resistance  of  long-distance 
runners  and  swimmers. 

The  Respiratory  Interchange  at  Low  Altitudes. — Pres- 
sures higher  than  the  atmospheric  must  be  endured  by  all 
those  persons  who  are  engaged  in  the  construction  of  tunnels 
and  piers  for  bridges  or  in  deep-sea  diving.  It  may  then 
become  necessary  to  fill  the  compartments  in  which  these 
men  are  working  with  compressed  air  so  as  to  balance  the 
pressure  of  the  water.  At  a  depth  of  34  feet  the  air  pressure 
must  be  twice  as  great  as  the  atmospheric,  and  at  68  feet 
three  times  as  great;  in  fact,  pressures  varying  between  four 
and  five  atmospheres  are  often  required  in  work  of  this  kind. 
In  order  to  become  accustomed  to  these  high  pressures,  the 
person  usually  passes  through  several  chambers,  in  which 
the  air  is  held  under  increasingly  greater  pressures.  The 
same  procedure  should  be  followed  in  leaving  the  place 
of  high  pressure,  because  if  the  decompression  is  accomplished 
too  hastily,  the  person  is  prone  to  develop  the  so-called 
caisson  disease,  or,  as  the  workmen  call  it,  the  "bends." 
As  the  name  suggests,  one  of  its  principal  symptoms  is 
severe  muscular  pain  which  is  associated  with  muscular 
spasms;  in  fact,  it  may  also  be  characterized  by  a  paralysis 
of  certain  groups  of  muscles.  The  name  usually  applied 
to  the  latter  symptom  is  diver's  palsy. 

The  principal  danger  associated  with  work  under  high 
degrees  of  barometric  pressure,  does  not  seem  to  lie  in  a  dis- 
turbance of  the  interchange  of  the  oxygen  or  carbon  dioxid, 
but  in  that  of  the  nitrogen.  Obviously,  the  amount  of  this 
gas  present  in  the  system  must  be  considerably  increased  at 
lower  levels,  for  as  this  gas  does  not  possess  a  distinct  respira- 
tory function,  its  molecules  must  be  held  in  the  body-fluids 
in  physical  solution.  Further,  the  number  of  the  molecules 
of  nitrogen  in  these  fluids  must  be  much  greater  at  low  levels 
than  at  high  levels.  If  the  person  now  passes  from  a  place 
of  high  barometric  pressure  (low  level)  into  one  in  which  the 


THE    CHEMISTRY    OF    RESPIRATION  209 

pressure  is  less  (higher  level),  these  molecules  of  nitrogen 
must  seek  to  escape  in  the  direction  of  least  resistance.  No 
disturbances  of  function  result  when  the  decompression 
is  accomplished  in  a  very  gradual  manner,  whereas  a  quick 
decompression  invariably  causes  these  molecules  to  break 
directly  through  the  tissues.  In  this  way,  certain  ganglion 
cells  and  nerve  fibers  are  frequently  destroyed  which  sub- 
serve muscular  motion.  A  paralysis  of  the  muscles  inner- 
vated by  these  cells  must  be  the  result  of  such  an  injury. 


CHAPTER  XX 
THE  NERVOUS  REGULATION  OF  RESPIRATION 

The  Respiratory  Center. — The  activity  of  the  different 
muscles  of  respiration  is  controlled  by  a  special  group  of 
nerve  cells,  situated  in  the  medulla  oblongata.  This  center 
communicates  with  the  aforesaid  muscles  by  means  of  a 
number  of  efferent  nerves,  and  is  itself  connected  with  differ- 
ent sense-organs  by  numerous  afferent  paths.  The  principal 
efferent  channels  are  formed  by  the  phrenic  nerves  which 
control  the  action  of  the  diaphragm,  the  most  important 
muscle  of  inspiration.  It  will  be  seen,  therefore,  that  the 
general  arrangement  of  the  mechanism  controlling  the 
respiratory  movements  is  practically  the  same  as  that  regulat- 
ing the  action  of  the  heart  and  the  caliber  of  the  arteries. 
The  cells  of  this  center  discharge  impulses  at  regular  inter- 
vals which  cause  certain  groups  of  muscles  to  contract, 
thereby  instigating  the  inspiratory  movement  This  phase 
of  muscular  activity  is  immediately  followed  by  the  largely 
passive  expiratory  movement. 

In  further  analysis  of  this  subject-matter,  our  attention 
should  next  be  directed  to  the  cause  of  the  orderly  sequence 
of  the  respiratory  movements  and  the  variations  in  the 
frequency  and  depth  of  these  movements  resulting  in  conse- 
quence of  afferent  stimuli.  The  lungs  are  essentially  a  pair 
of  bellows  which  may  be  expanded  either  at  a  fast  or  a  slow 
rate,  and  either  at  regular  or  irregular  intervals.  Their 
expansion,  as  has  just  been  stated,  is  occasioned  by  impulses 
discharged  by  the  center.  Whenever  these  impulses  are 
blocked,  respiration  must  cease.  Accordingly,  it  may  justly 
be  assumed  that  the  action  of  the  respiratory  center  is  auto- 
matic, i.e.,  it  takes  place  in  consequence  of  certain  inherent 
stimuli.  This  primary  automatism,  however,  is  subject  to 
all  sorts  of  influences  and  in  a  much  greater  degree  than  that 

210 


NERVOUS    REGULATION    OF    RESPIRATION  211 

of  the  cardiac  mechanism.  This  conclusion  is  justified 
upon  the  ground  that  the  activity  of  the  heart  continues 
even  after  its  connections  with  the  central  nervous  system 
have  been  destroyed. 

The  Regulation  of  the  Respiratory  Movements. — The 
foregoing  discussion  must  have  shown  that  the  variations  in 
the  frequency  and  depth  of  the  successive  respiratory  move- 
ments originate  in  changes  in  the  number  and  character  of 
the  efferent  impulses  discharged  by  the  center.  The  only 
question  that  remains  to  be  answered  as  yet,  pertains  to  the 
nature  of  the  cause  producing  these  differences  in  the  func- 
tional .capacity  of  the  center.  Experimentation  has  proved 
that  the  chief  factor  is  the  quality  of  the  blood,  and,  natu- 
rally, the  only  characteristic  that  need  be  considered  in  this 
connection  is  its  oxygen  and  carbon  dioxid  content.  It  is 
easily  conceivable  that  an  augmentation  of  the  respiratory 
movements  must  result  whenever  the  percentage  of  the 
former  gas  is  diminished  or  that  of  the  latter  increased. 
Animal  experimentation  again  has  shown  that  the  carbon 
dioxid  is  the  more  important  factor  of  the  two,  because  the 
slightest  possible  increase  in  the  percentage  of  this  gas  in 
the  blood  gives  rise  to  a  very  decided  increase  in  the  rate 
and  depth  of  the  respiratory  movements.  Thus,  carbon 
dioxid  really  plays  the  part  of  a  specific  stimulant  of  the 
respiratory  center.  When  its  percentage  in  the  blood  is 
raised,  the  constituents  of  the  repiratory  center  immediately 
react  toward  this  change  by  evoking  more  frequent  and 
powerful  contractions  of  the  respiratory  muscles. 

The  second  question  pertains  to  the  nature  of  the 
mechanism  by  means  of  which  the  individual  respiratory 
movements  are  made  to  retain  a  definite  length  and  to  follow 
one  another  at  perfectly  regular  intervals.  During  its  descent 
through  the  neck  and  thorax  the  vagus  nerve  gives  off  several 
branches.  Two  of  these,  namely,  the  superior  and  inferior 
laryngeal  nerves,  innervate  the  larynx  and  adjoining  parts, 
whereas  its  third  lateral  passes  to  the  musculature  of  the 
bronchi  and  larger  bronchioles.  It  appears  that  the  end- 
organs  of  these  fibers  are  acted  upon  in  a  mechanical  way 
by  the  distention  of  these  tubules.  Afferent  impulses  arise 


212  KESPIRATION 

in  consequence  of  these  impacts  which  are  conveyed  to  the 
respiratory  center,  where  they  inhibit  either  the  inspiratory 
or  the  expiratory  movement.  In  this  way,  the  expansion 
of  the  lungs  gives  rise  to  certain  afferent  impulses  which  stop 
this  movement  at  a  precise  moment  and  allow  expiration 
to  set  in.  Likewise,  the  deflation  of  these  organs  incites 
the  succeeding  inspiration.  This  check-system  is  usually 
designated  as  the  self-regulation  of  respiration  by  means  of 
the  vagus  nerve. 

The  Co-ordination  between  the  Respiratory  and  Circula- 
tory Mechanisms. — It  must  be  evident  that  the  interchange 
of  the  gases  in  the  lungs  cannot  satisfactorily  fulfill  its  purpose 
unless  a  sufficient  amount  of  non-aerated  blood  is  constantly 
diverted  into  the  pulmonary  circuit.  In  last  analysis, 
therefore,  it  will  be  found  that  it  is  the  interchange  between 
the  blood  and  the  cells  of  the  tissues  which  must  be  protected 
above  everything  t  else ;  and,  obviously,  this  interchange 
cannot  be  accomplished  if  the  circulation  is  at  all  impaired. 
Accordingly,  an  intense  ventilation  of  the  lungs  called  forth 
by  an  accumulation  of  waste  products,  should  invariably 
be  associated  with  an  increase  in  the  rate  and  force  of  the 
heart  beat,  as  well  as  an  increased  vascular  tonus.  These 
factors  combined  enhance  internal  respiration. 

It  is  a  matter  of  common  experience  that  the  correlation 
between  the  circulatory  and  respiratory  mechanisms  is  a 
very  close  one.  We  well  know  that  even  a  very  slight  muscu- 
lar effort,  such  as  is  required  to  keep  the  body  in  the  standing 
position,  is  accompanied  by  a  greater  gas  exchange  and 
hence,  also  by  a  greater  cardiac  and  respiratory  activity. 
Vigorous,  yet  not  excessive,  muscular  exercise  will  increase 
this  exchange  tenfold,  whereas  rest  and  sleep  will  reduce  it  to 
one-half  its  ordinary  value.  All  forms  of  exercise,  when 
performed  in  a  cool  and  relatively  dry  atmosphere,  cause  us  tc_ 
dissipate  a  larger  amount  of  heat  which  must  immediately 
be  compensated  for  by  a  greater  production  of  heat.  These 
changes  necessitate  a  more  intense  gas  exchange  and  quick- 
ened circulation. 

Breathlessness. — As  is  true  of  other  mechanisms,  the 
continued  overwork  of  the  respiratory  organs  finally  leads 


NERVOUS    REGULATION    OF    RESPIRATION  213 

to  fatigue.  The  symptoms  associated  therewith  may  be 
objective  and  subjective,  i.e.,  they  may  betray  themselves  by 
peculiar  sensations  in  the  respiratory  muscles  and  parts 
moved  by  them  or  by  a  more  general  condition  of  discomfort. 
It  is  obvious,  however,  that  they  cannot  be  perfectly  local- 
ized, because  a  fatigue  of  the  respiratory  muscles,  exclusive 
of  other  parts,  is  scarcely  possible  under  ordinary  circum- 
stances. Rated  physiologically,  fatigue  is  merely  a  signal 
set  by  nature  against  the  continuance  of  excessive  exercise 
and  the  greater  activity  of  the  cardiac  and  respiratory 
mechanisms  invariably  associated  therewith.  When  this 
danger  signal  is  not  heeded,  nature  makes  use  of  another 
safety  appliance,  namely,  that  of  establishing  shortness  of 
breath.  Hence,  this  condition  may  be  employed  as  a  physi- 
ological means  for  ascertaining  the  "dose"  of  exercise  that 
may  safely  be  prescribed. 

Breathlessness,  is  merely  an  advanced  stage  of  fatigue, 
which  would  finally  culminate  in  complete  dyspnoea  and 
exhaustion.  It  is  characterized  by  a  feeling  of  distress,  indi- 
cative of  a  failure  of  the  gas  exchange  and  circulation.  The 
inspiratory  movements  become  long  and  forced,  whereas 
expiration  is  not  materially  prolonged.  In  order  to  bring 
this  condition  about  in  a  non-fatigued  animal,  it  is  necessary 
to  subject  it  to  a  form  of  exercise  which  will  cause  it  to  expend 
an  unusual  amount  of  energy  in  a  comparatively  brief  period 
of  time.  But,  naturally,  the  measure  of  work  actually 
required  to  produce  shortness  of  breath,  differs  with  the 
state  of  the  animal.  Fatigue  favors  its  appearance,  while  rest 
retards  its  onset.  A  simple  way  of  producing  this  condition 
is  to  climb  two  flights  of  stairs  in  the  time  of  one  minute 
or  four  flights  of  stairs  in  two  minutes.  Supposing  that  the 
weight  of  the  person  is  75  Kg,  and  the  total  height  of  the 
stairs  20  m.,  he  will  have  executed  75  X  20  =  1500  kilo- 
grammeters  of  work  in  the  course  of  two  minutes.  This 
work  may  be  rated  as  considerable,  because  it  corresponds  to 
the  raising  of  a  weight  of  50  kg.  thirty  times  to  the  height  of 
1m.  In  order  to  accomplish  this  amount  of  work  an  unusual 
degree  of  effort  is  required,  and  extreme  effort  invariably 
predisposes  to  breathlessness. 


214  RESPIRATION 

We  have  just  noted  that  the  respiratory  requirements  of 
the  body  correspond  very  closely  to  the  percentage  of  carbon 
dioxid  in  the  blood.  Accordingly,  breathlessness  must  find 
its  cause  in  a  superfluity  of  this  gas  in  the  system.  Its 
accumulation  may  be  explained  upon  the  basis  of  either  an 
excessive  production  or  an  impaired  elimination.  We  well 
know  that  certain  persons  suffer  from  breathlessness  even 
after  the  slightest  exertion  and  hence,  the  difficulty  does 
not  seem  to  lie  in  an  excessive  production,  but  in  a  reduced 
elimination  of  this  gas.  Further,  inasmuch  as  the  ventilation 
ordinarily  established  in  the  lungs,  is  more  than  ample  to 
allow  for  even  a  very  considerable  increase  in  the  gas  ex- 
change, this  respiratory  difficulty  may  justly  be  referred  to 
an  inadequate  reaction  of  the  circulatory  mechanism.  This 
inability  on  the  part  of  the  heart  and  bloodvessels  to  take 
care  of  the  increased  respiratory  activity,  must  lead  to  a 
passive  congestion  of  the  capillaries  of  the  lungs  and  in- 
directly also  to  a  diminution  in  the  gas  exchange. 

It  is  a  well  known  fact  that  persons  afflicted  with  lesions 
of  the  valves  of  the  heart,  are  made  " breathless"  by  even 
very  mild  forms  of  exercise.  In  fact,  this  condition  fre- 
quently arises  without  any  exertion  whatever  in  consequence 
of  periodic  depressions  of  the  cardiac  musculature.  It  is 
usually  designated  as  "cardiac  dyspnoea,"  and  finds  its 
origin  in  a  stagnation  of  the  blood  in  the  pulmonary  vessels 
and  consequent  lowering  of  the  aeration  of  the  tissues. 

Breathing  Exercises. — It  is  a  matter  of  common  experi- 
ence that  a  person  who  has  become  " stale"  from  lack  of  exer- 
cise, cannot  endure  even  a  very  moderate  type  of  muscular 
effort  without  suffering  from  breathlessness.  This  condition, 
however,  may  be  gradually  overcome  by  physical  train- 
ing. Provided  the  person  is  otherwise  perfectly  healthy,  a 
decided  improvement  should  be  noted  from  day  to  day  as 
the  circulatory  mechanism  adapts  itself  to  the  more  vigorous 
work.  A  similar  kind  of  adaptation  is  brought  about  by 
the  process  of  "warming  up,"  consisting  in  a  brief  indulgence 
in  muscular  exercise  immediately  before  the  real  test  is  to  be 
undertaken.  The  circulation  is  thereby  put  in  the  best 
possible  condition  for  the  more  vigorous  work  that  is  to  follow. 


NERVOUS    REGULATION    OF    RESPIRATION  215 

The  discussions  pertaining  to  the  mechanics  of  respiration 
must  have  shown  that  the  only  safe  way  to  increase  the  size 
and  capacity  of  the  chest,  is  to  indulge  in  those  moderate 
exercises  which  will  require  a  gradually  increased  gas  ex- 
change. This  statement  implies  that  breathing  exercises 
as  such  are  insufficient  and  ma}^  in  fact  prove  harmful,  owing 
to  the  excessive  ventilation  of  the  lungs  instituted  thereby. 
Nature  tends  to  prevent  too  copious  a  supply  of  oxygen 
(apncea)  and  depletion  of  carbon  dioxid  (acapnia)  by  in- 
hibiting the  respiratory  movements.  Furthermore,  periodic 
increases  in  the  ventilation  of  the  lungs  are  already  provided 
for,  because  the  normal  person  is  involuntarily  forced  to 
take  a  deep  breath  at  relatively  short  intervals.  It  will  be 
seen,  therefore,  that  the  respiratory  mechanism  is  self- 
regulatory,  and  that  a  greater  efficiency  can  only  be  im- 
parted to  it  indirectly  by  augmenting  the  gas  exchange 
through  exercise.  Do  not  put  the  cart  before  the  horse  and 
endeavor  to  increase  internal  respiration  by  voluntary  deep 
breathing  without  general  muscular  exercise. 

The  respiratory  movements  should  be  adjusted  in  such  a 
way  that  all  portions  of  the  lungs  are  expanded  in  an  equal 
measure,  because  it  is  conceivable  that  a  continued  diaphrag- 
matic type  of  respiration  may  give  rise  to  a  poor  ventilation 
of  their  apical  areas.  Thus,  it  is  commonly  believed  that  the 
lack  of  movement  in  the  upper  thorax  is  chiefly  responsible 
for  the  fact  that  as  many  as  80  per  cent,  of  the  cases  of  pul- 
monary tuberculosis  show  a  primary  infection  of  the  apical 
lobes.  Exercises  of  support  and  suspension  are  not  so  well 
thought  of  as  those  of  walking,  running,  and  wrestling, 
because  they  tend  to  hinder  the  action  of  the  respiratory 
muscles.  Thus,  mountaineers  have  large  chests,  because 
they  increase  their  respiratory  needs  in  the  most  physiological 
manner.  They  respire  upon  a  high  base-level  of  air  with 
the  chest  always  well  rounded,  the  pulmonary  vessels  well 
distended,  and  the  heart  in  a  state  of  favorable  diastolic 
expansion. 


PART  IV 
NUTRITION 

CHAPTER  XXI 
SECRETION 

General  Consideration. — Inasmuch  as  living  matter5  is 
constantly  called  upon  to  yield  energy,  it  must  be  placed  in 
possession  of  certain  chemical  compounds  from  which  it 
may  derive  this  energy.  Any  material,  the  physico-chemical 
constitution  of  which  may  be  altered  by  living  matter  so  as 
to  liberate  energy,  is  known  as  a  food.  Naturally,  such 
material  may  be  reduced  immediately  upon  coming  in 
contact  with  the  cell  or  may  be  stored  in  it  for  some  time 
until  its  metabolic  needs  require  an  additional  supply  of 
fuel.  Every  living  cell,  therefore,  must  present  two  nutri- 
tive phases :  namely,  one  during  which  it  builds  up  its  elemen- 
tary substances,  and  one  during  which  it  again  splits  them 
into  their  simplest  components  and  discharges  them  under 
an  evolution  of  energy.  The  former  process  is  known  as 
assimilation  and  the  latter  as  dissimilation. 

These  two  stages,  however,  do  not  constitute  the  entire 
life  history  of  the  food,  because  its  assimilation  must  be  pre- 
ceded by  certain  processes  purposing  to  render  the  nutritive 
material  available  to  the  cell.  Quite  similarly,  dissimilation 
must  be  followed  by  certain  processes  enabling  the  cell  to 
rid  itself  of  those  substances  which  have  been  rendered  useless 
in  the  course  of  the  oxidations.  These  changes  are  most 
clearly  revealed  by  the  lower  forms,  because  every  free-- 
living cell  is  in  possession  of  certain  means  empowering  it  to 
catch  the  food  and  to  digest  it  before  it  is  actually  assimilated. 
Likewise,  its  activities  are  adjusted  in  such  a  way  that  cer- 
tain portions  of  its  body  wholly  subserve  the  process  of 
excretion.  In  many  instances,  however,  the  digestion  and 

216 


SECRETION  217 

excretion  of  the  nutritive  material  are  completed  within  the 
boundaries  of  a  single  cell,  so  that  a  division  of  labor  is  not 
easily  apparent,  although  doubtlessly  present. 

In  the  higher  animals,  digestion  is  accomplished  through 
the  instrumentality  of  a  set  of  specialized  organs,  comprising 
various  tubular  receptacles  and  glands.  After  its  simplifica- 
tion the  food  is  absorbed  and  conveyed  to  the  cells  of  the 
different  tissues  to  be  assimilated.  This  process  is  followed 
by  that  of  dissimilation,  the  end-products  of  the  food  being 
finally  removed  from  the  body  by  special  organs,  constituting 
the  group  of  the  excretory  organs.  It  is  to  be  noted,  there- 
fore, that  digestion  in  the  highest  forms  is  extracellular  in 
its  character,  because  the  alimentary  canal  in  which  the 
cleavage  of  the  food  is  accomplished,  lies  really  outside  the 
body  as  far  as  the  mass  of  the  tissue  cells  is  concerned. 
Communication  is  established  between  the  source  of  the 
nutritive  supply  and  the  tissues  by  means  of  two  carriers: 
namely,  the  blood  and  lymph. 

The  life-history  of  the  food  begins  with  its  ingestion  and 
ends  with  its  excretion.  The  sum  of  the  chemical  changes 
taking  place  in  it  during  this  interim  while  traversing  the 
body,  is  usually  designated  as  metabolism,  a  term  which  really 
means  "transformation  of  matter."  But,  since  the  proc- 
esses of  digestion  take  place  in  a  membranous  tube  which 
really  represents  an  invagination  of  the  body-surface  and 
lies,  therefore,  outside  the  body  proper,  it  may  not  be  quite 
correct  to  include  ingestion,  digestion  and  excretion  under 
this  heading.  Accordingly,  digestion  might  be  more  properly 
regarded  as  a  pre-metabolic  stage,  and  excretion  as  a  post- 
metabolic  phase.  All  these  processes  might  then  be  classified 
in  the  following  manner  under  the  general  term  of  nutrition : 

..  I  Mastication 

Ingestmn       j  Deglutition 

T^.      , .  J  Mechanical  processes 

Digestion       j  Chemical  presses 


Nutrition 


Absorption 

iv/r  +  K  r  f  Anabolism — cellular  assimilation 

1  1  Catabolism— cellular  dissimilation 

,,  J  Mechanical  processes 

Excretion  |  Chemical  processes 


218 


NUTRITION 


The  Purpose  of  Digestion. — Food  is  a  mixture  of  different 
substances.  A  piece  of  bread  or  meat  does  not  consist  of  a 
single  chemical  entity,  but  of  several  which  are  usually 
designated  as  foodstuffs.  These  foodstuffs  may  be  arranged 
in  the  following  order: 


Food- 
stuffs 


Inorganic 


Water 

Salts 


Chloride 
Phosphate 
Sulphate 
Carbonate 


of  sodium  and  potassium 


Phosphate 
Carbonate 


Organic 


>  of  calcium  and  magnesium 
Proteins — nitrogenous 


Carbohydrates 


The  salts  as  such  are  not  oxidized  and  cannot,  therefore, 
be  considered  as  a  direct  source  of  energy.  Their  chief 
purpose  is  to  maintain  the  reaction  of  the  fluids  of  the  body, 
and  to  serve  as  tissue  builders.  In  the  absence  of  these 
salts,  it  would  be  quite  impossible  to  form  efficient  media  for 
diffusion  and  osmosis. 

The  water,  salts,  and  simple  sugars,  such  as  the  dextrose 
of  grapes  and  the  levulose  of  fruits,  need  not  be  acted  upon 
beforehand,  because  they  are  already  in  so  simple  a  form 
that  they  can  traverse  the  lining  of  the  intestine  without 
cleavage.  Cane-sugar,  on  the  other  hand,  is  not  a  native 
compound  and  cannot  be  assimilated  as  such,  although  it  is 
soluble  and  diffusible.  The  remaining  foodstuffs,  inclusive 
of  the  more  complex  carbohydrates,  must  first  be  simplified 
before  they  can  enter  the  absorbing  channels  of  the  body. 
Thus,  it  may  be  said  that  the  purpose  of  digestion  is  to  split 
the  complex  molecules  of  the  foodstuffs  into  smaller  ones, 
because  only  in  this  form  are  they  able  to  pass  through  animal 
membranes,  such  as  the  mucosa  of  the  stomach  and  intestine. 
Having  reached  the  other  side  of  this  limiting  membrane, 
they  find  their  way  into  either  the  capillaries  of  the  portal 


SECRETION  219 

circulation  or  the  lacteals  of  the  lymphatic  system.  Eventu- 
ally they  attain  the  general  circulatory  channels,  and  are 
finally  assimilated  by  the  cells  of  the  various  tissues.  Hence, 
it  may  be  concluded  that  the  process  of  digestion  strives  to 
render  the  ingested  material  dialyzable  through  the  limiting 
membranes  of  the  body. 

In  order  to  accomplish  this  end,  the  body  must  be  in 
possession  of  two  things :  namely,  a  receptacle  in  which  these 
reductions  can  be  brought  to  completion,  and  certain  chemi- 
cal substances  able  to  institute  this  cleavage.  The  newly 
ingested  food  is  forced  through  the  different  segments  of  the 
alimentary  canal  by  the  muscular  action  of  its  wall.  During 
its  passage  through  this  membranous  tube  it  is  subjected  to 
the  action  of  diverse  glandular  products  which  separate  its 
useful  from  its  useless  constituents.  The  chemical  factor 
concerned  with  the  digestion  of  the  foodstuffs,  is  furnished 
by  several  glands  which  are  either  incorporated  in  the  wall 
of  this  tubular  passage  or  form  relatively  independent 
masses  of  tissue  in  its  immediate  vicinity.  Each  gland  pro- 
duces one  or  several  active  principles,  possessing  specific 
chemical  influences  upon  the  different  foodstuffs.  Thus, 
while  one  principle  may  be  peculiarly  adapted  to  split  the 
protein  molecule,  another  may  possess  a  selective  action 
upon  the  carbohydrates  or  the  fats.  Furthermore,  the 
sphere  of  action  of  these  agents  is  considerably  increased  by 
the  fact  that  they  are  finely  subdivided  and  suspended  in  a 
watery  medium  which  allows  them  to  form  contact  with  very 
large  amounts  of  food. 

These  watery  media,  embracing  these  specific  digestive 
agents  as  well  as  a  certain  amount  of  extraneous  material,  are 
known  as  secretions.  A  general  definition  of  a  secretion, 
however,  cannot  be  based  upon  its  consistency,  because  many 
secretions,  such  as  the  product  of  the  sebaceous  glands,  are 
not  fluids.  For  this  reason,  it  is  best  to  define  a  secretion 
merely  as  a  cellular  product  which  is  of  further  use  to  the 
body,  while  an  excretion  is  a  cellular  product  which  is  of  no 
further  use  to  the  body. 

Classification  of  the  Secretions. — The  general  arrangement 
of  the  elements  composing  a  secretory  gland,  is  indicated  in 


220  NUTRITION 

Fig.  91.  We  observe  here  that  a  number  of  cells  are  grouped 
around  the  bulbular  extremity  of  a  small  duct,  their  outer 
surfaces  lying  in  relation  with  a  net-work  of  capillaries  from 
which  the  different  constituents  of  the  secretion  are  derived. 
A  colony  of  cells  of  this  kind  is  designated  as  an  acinus. 
Many  acini  are  combined  into  a  lobule,  and  many  lobules 

into  a  lobe.  The  gland  as 
a  whole  may  embrace  a 
large  number  of  lobes. 

Secretions  should  be 
classified  first  of  all  as  ex- 
ternal and  internal.  The 
" external"  ones  are  poured 
upon  an  open  surface  of  the 
body,  whereas  the  internal 
ones  are  passed  directly 

FIG.  91. — Diagrammatic  represen-  into  the  blood-  Or  lymph- 
tation  of  an  acinus.  '  Z>,  duct;  S.  stream.  The  glands  of 
secretory  cells;  L,  lymph  space;  C,  ,  .. 

blood  capillaries.  external    secretion    are    in 

possession  of  clearly  differ- 
entiated membranous  tubes  by  means  of  which  their  products 
are  conveyed  to  a  place  situated  at  times  at  a  considerable 
distance  from  them.  The  glands  of  internal  secretion,  on 
the  other  hand,  do  not  possess  a  distinct  duct.  In  this 
group  belong  the  pineal  and  pituitary  bodies,  the  thyroid 
and  parathyroids,  the  thymus,  pancreas,  liver,  suprarenal 
capsules,  testes,  and  ovaries.  We  know  that  these  struc- 
tures furnish  an  important  product,  because  their  removal  is 
followed  in  several  of  these  instances  by  the  death  of  the 
animal.  Among  the  glands  with  very  conspicuous  ducts 
might  'be  mentioned  those  which  furnish  the  sweat,  milk, 
saliva,  pancreatic  juice,  and  bile. 

It  is  also  to  be  noted  that  certain  organs,  such  as  the  liver, 
pancreas,  testes,  and  ovaries,  produce  an  external  as  well 
as  an  internal  secretion.  The  pancreas,  for  example,  gives 
rise  to  the  pancreatic  juice  which  is  poured  into  the  duodenum 
through  the  duct  of  Wirsung  and  possesses  a  most  important 
chemical  action  upon  the  food  in  this  particular  segment  of 
the  intestine.  In  addition,  however,  the  spaces  in  between 


SECRETION  221 

the  different  acini  of  this  gland  contain  cells  of  an  entirely 
different  character  which  furnish  an  internal  product  having 
to  do  with  the  metabolism  of  the  sugars.  It  is  a  well  recog- 
nized fact  that  the  destruction  of  the  pancreas  by  disease 
or  injury  and  subsequent  loss  of  the  pancreatic  juice,  is 
invariably  followed  by  a  severe  impairment  of  the  digestive 
processes,  but  the  most  disturbing  symptoms  then  develop- 
ing are  those  indicating  a  derangement  in  the  oxidation  of  the 
sugars.  This  complex  of  symptoms  constitutes  the  disease 
of  diabetes  mellitus.  Likewise,  the  liver  furnishes  an  external 
excretion,  the  bile,  which  is  concerned  with  the  digestion  of 
the  fats,  and  secondly,  a  certain  internal  product  which 
enables  the  cells  of  this  organ  to  store  the  absorbed  sugar  in 
the  form  of  glycogen. 

The  Active  Principle  of  a  Secretion. — The  secretions  with 
which  we  are  most  intimately  concerned  at  this  time,  have  to 
do  with  the  cleavage  of  the  different  foodstuffs.  This 
group  of  the  digestive  secretions  includes  the  saliva,  gastric 
juice,  pancreatic  juice,  bile,  and  intestinal  juice.  All  of  them 
possess  the  power  of  simplifying  the  complex  molecules  of 
the  food,  their  peculiar  action  being  dependent  upon  the 
presence  of  a  catalytic  agent,  commonly  termed  a  ferment. 
It  has  been  known  for  a  long  time  that  very  minute  quantities 
of  certain  substances  possess  the  power  of  instigating  a 
chemical  reaction,  although  they  themselves  are  not  trans- 
formed nor  destroyed  in  the  course  of  this  process.  These 
bodies  are  commonly  designated  as  ferments.  The  example 
usually  cited  to  illustrate  their  action,  is  the  splitting  of  sugar 
into  carbon  dioxid  and  alcohol  under  the  influence  of  living 
yeast  cells.  Because  of  the  "boiling  up"  of  this  solution, 
this  process  has  been  designated  as  fermentation,  a  term 
derived  from  the  Latin  verb:  "fervere." 

Experimentation,  however,  has  proved  that  ferment-action 
does  not  require  the  presence  of  living  cells  and  hence,  the 
early  classification  of  ferments  into  dead  and  living  or  unor- 
ganized and  organized  ferments  is  no  longer  tenable.  We 
now  know  that  the  yeast  cells  may  first  be  killed  by  heat  and 
other  means  without  destroying  their  power  of  instigating 
the  aforesaid'  reaction.  Accordingly,  the  term  ferment 


222  NUTRITION 

should  be  employed  only  in  a  general  sense  to  indicate  all 
bodies  of  this  kind,  although  the  term  enzyme  may  be  re- 
tained when  reference  is  made  to  such  active  agents  as  the 
ptyalin  of  the  saliva  or  the  pepsin  of  the  gastric  juice.  It 
should  be  remembered,  however,  that  their  action  is  the 
same  in  principle. 

As  has  been  stated  above,  ferments  are  not  destroyed 
during  the  reaction  incited  by  them  and  hence,  the  medium 
must  still  contain  them  after  the  entire  process  has  been 
completed.  It  must  be  granted,  however,  that  their  re- 
covery is  not  easily  accomplished.  It  should  also  be  noted 
that  ferments  act  in  very  minute  quantities  and  are  specific  in 
their  nature,  i.e.,  they  cannot  incite  several  different  cleav- 
ages. These  agents  are  not  contained  in  the  cells  of  the 
gland  as  such  but  in  a  dormant  and  more  elementary  state. 
Thus,  we  find  that  the  pepsin  of  the  gastric  juice  is  stored  in 
the  form  of  the  inactive  pepsinogen,  and  changes  into  the 
powerful  enzyme  only  after  it  has  been  discharged  into  the 
duct  and  has  been  brought  in  contact  with  the  hydrochloric 
acid  of  this  secretion. 

The  fact  that  these  enzymes  do  not  possess  a  universal 
action,  again  calls  to  our  minds  the  general  purpose  of  diges- 
tion, which  is  to  cleave  the  complex  molecules  of  the  food- 
stuffs into  smaller  ones.  This  simplification  renders  the 
otherwise  useless  nutrients  dialyzable.  Food  presents  a 
manifold  appearance  and  composition,  although  always 
embracing  one  or  several  of  the  food-stuffs  enumerated 
above.  By  ferment-action  its  useless  constituents  are  sepa- 
rated from  those  possessing  an  absolute  value  to  the  body. 
Only  the  latter  are  rendered  available  for  absorption  by 
changes  which,  after  all,  are  quite  simple,  because  they  con- 
sist merely  in  cleavages.  Thus,  it  may  be  stated  that  en- 
zymes are  selective  agents  which  ascertain  the  character  of 
the  material  that  should  be  allowed  to  pass  into  the  body. 
In  accordance  with  the  type  of  the  food-stuff  acted  upon, 
they  are  generally  classified  as  protein-splitting  or  proteolytic, 
fat-splitting  or  lipolytic,  and  starch-splitting  or  amylolytic. 


CHAPTER  XXII 
SALIVARY  DIGESTION 

The  Alimentary  Canal. — The  length  of  the  alimentary 
canal  may  be  estimated  at  about  five  to  six  times  that  of  the 
entire  body.  It  begins  above  with  the  cavity  of  the  mouth, 
and  terminates  below  at  the  anus,  traversing  during  its 
course  the  entire  length  of  the  thoracic  and  abdominal  cavi- 
ties. In  the  neck  and  thorax,  it  pursues  a  rather  straight 
course,  while  in  the  abdominal  cavity  it  is  repeatedly  wound 
upon  itself  so  as  to  form  a  number  of  closely  packed  coils. 
The  general  arrangement  of  this  musculo-membranous 
tube  varies  somewhat  in  different  mammals  in  accordance 
with  the  type  of  the  food  consumed  by  them.  Thus,  the 
carnivorous  or  meat-eating  animals  are  characterized  by  a 
short  large  intestine  and  very  prominent  small  intestine. 
In  the  herbivora  or  plant-eating  animals  this  relationship 
is  reversed,  their  abdominal  cavity  being  fully  occupied  by 
the  cavernous  large  intestine.  Man  belongs  to  the  class  of 
the  omnivorous  animals  which  occupies  in  this  regard  a 
position  intermediate  between  the  groups  just  mentioned. 

This  difference  in  the  structure  of  this  part  possesses  a 
definite  functional  value,  because  the  cleavage  and  absorption 
of  the  proteins  are  accomplished  almost  exclusively  in  the 
upper  intestinal  tract,  and  require  only  a  comparatively 
brief  period  of  time  for  their  completion.  The  reduction 
of  the  grasses,  on  the  other  hand,  often  consumes  several 
days,  because  their  cellulose  investments  must  first  be 
broken  up  before  their  nutrient  constituents  can  be  attacked. 
The  swelling,  erosion,  and  maceration  of  the  plants  is  ac- 
complished chiefly  in  the  beginning  portion  of  the  large 
intestine,  the  cecum.  In  all  three  groups  of  animals,  how- 
ever, the  alimentary  canal  consists  of  a  series  of  clearly 
differentiated  segments  which  are  known  as  the  mouth, 
pharynx,  oesophagus,  stomach,  and  small  and  large  intestines. 

223 


224  NUTRITION 

The  Wall  of  the  Alimentary  Canal. — In  general,  it  may  be 
said  that  the  wall  of  the  alimentary  tract  is  composed  of 
three  layers  of  tissue:  namely,  a  mucous  lining,  an  inter- 
mediate coat  of  muscle  tissue,  and  an  outer  serous  covering. 
These  layers  present  the  following  characteristics : 

(a)  the  mucosa  is  a  soft,  velvety  lining  membrane,  situ- 
ated upon  a  thin  lamella  of  connective  tissue.  It  contains 
glandular  structures  which  secrete  either  a  true  digestive 
fluid  or  merely  mucus  for  purposes  of  lubrication; 

(6)  the  muscular  coat  consists  of  smooth  muscle  cells  and  a 
framework  of  connective  tissue.  These  cells  are  arranged 
circularly  around  the  lumen  of  the  canal  as  well  as  longitu- 
dinally to  it.  The  circular  ones  attach  themselves  in  the 
form  of  a  heavy  layer  to  the  mucosa.  In  accordance  with 
this  general  arrangement,  their  contraction  gives  rise  to  a 
constriction  of  the  lumen  of  the  canal,  while  that  of  the  more 
external  longitudinal  cells  shortens  this  tube.  The  muscular 
movements  noted  along  the  alimentary  canal  are  peculiar 
insofar  as  they  do  not  consist  of  simple  contractions  of  the 
circular  fibers,  but  of  a  combined  action  of  the  two  sets  of 
muscle  cells.  The  product  of  their  activity  is  known  as  the 
peristaltic  wave.  It  is  to  be  observed  that  this  movement 
presents  itself  as  a  ring-like  constriction  which  is  preceded 
by  a  band-like  zone  of  relaxation.  Both  together  involve  the 
consecutive  segments  of  the  canal  in  the  form  of  a  wave. 
The  food  is  forced  ahead  of  the  constricting  area  in  the  direc- 
tion of  least  resistance  established  by  the  relaxation.  In  the 
stomach  we  also  find  a  certain  number  of  muscle  cells  which 
pursue  an  oblique  course  across  its  cardiac  portion.  This 
layer  plays  an  important  part  in  the  process  of  evacuation  of 
this  organ. 

(c)  the  serous  layer  or  peritoneum  envelops  the  external 
surfaces  of  the  digestive  tract  and  organs  associated  with  it, 
and  is  finally  reflected  upon  the  internal  surface  of  the  wall 
of  the  abdomen.  It  appears,  therefore,  in  the  form  of  a 
visceral  and  parietal  layer.  A  minute  study  of  the  course 
pursued  by  it  in  covering  the  abdominal  organs  cannot  be 
undertaken  at  this  time,  although  it  should  be  noted  that  it 
forms  two  very  important  folds  which  are  known  respectively 


SALIVARY    DIGESTION 


225 


as  the  great  omentum  and  the  mesentery.  The  former  is 
suspended  like  a  curtain  from  the  greater  curvature  of  the 
stomach  and  envelops  the  intestine  in  front  in  the  shape  of 
an  apron.  The  mesentery  is  a  duplicature  which  surrounds 
the  greater  part  of  the  small  intestine  in  the  form  of  a  sling, 
and  fixes  it  more  securely  to  the  posterior  wall  of  the  ab- 
dominal cavity.  Like  all  serous  membranes,  the  peritoneum 
is  moistened  with  a  lymph-like 
fluid  which  acts  as  a  lubricant. 
It  is  to  be  noted,  however, 
that  the  different  abdominal 
organs  are  packed  closely 
together,  so  that  no  air  is  left 
between  them.  But,  the  ali- 
mentary canal  itself  often 
contains  considerable  amounts 
of  air  which  have  either  been 
swallowed  with  '  the  food  or 
have  been  formed  in  the  course 
of  fermentations. 

The  Changes  in  the  Food 
Effected  in  the  Mouth. — Be- 
fore being  ingested,  the  food 
is  usually  subjected  to  certain 
procedures  which  render  it 
more  vulnerable  to  the  di- 
gestive juices.  The  process  of 
cooking,  for  example,  has  a 

twofold  purpose:  namely,  a  mechanical  one  and  a  chemical 
one.  It  destroys  the  cellulose  capsules  and  partitions  of  the 
vegetables,  and  lacerates  the  firm  fibrous  investments  of  the 
food  of  animal  origin,  such  as  meat.  Thus,  while  rice  and 
similar  foods  may  be  very  completely  digested  (80  per  cent.) 
without  having  been  cooked,  this  procedure  nevertheless 
aids  materially  in  the  cleavage  and  assimilation  of  their 
constituents.  Their  continued  boiling  disintegrates  the 
starch  granules,  splitting  them  into  an  infinite  number  of 
small  particles.  When  thus  finely  subdivided,  they  present 
a  much  larger  surface  to  the  digestive  juices.  Similar 

15 


FIG.  92. — Dissection  of  the  side 
of  the  face,  showing  the  salivary 
glands,  a,  sublingual  gland;  6, 
submaxillary  gland,  with  its  duct 
opening  on  the  floor  of  the  mouth 
beneath  the  tongue  at  d;  c, 
parotid  gland  and  its  duct,  which 
opens  on  the  inner  side  of  the 
cheek.  (After  Yeo.) 


226  NUTRITION 

processes  are  the  milling  of  grain,  and  the  ripening  of  fruits 
and  meats. 

After  its  entrance  into  the  mouth  the  food  undergoes  a 
twofold  reduction :  namely,  a  mechanical  one  and  a  chemical 
one.  The  former  process  is  called  mastication.  It  consists 
in  the  breaking  up  of  the  larger  pieces  of  food  into  smaller 
ones,  and  the  subsequent  amalgamation  of  the  finely  sub- 
divided .material  into  a  rounded,  softened  mass  which  is 
known  as  the  bolus.  The  chemical  action  taking  place  in 
this  cavity  is  restricted  to  the  starches  and  is  accomplished 
by  means  of  the  first  digestive  secretion,  the  saliva.  Its 
enzyme  ptyalin  is  the  active  principle  responsible  for  this 
cleavage. 

Mastication. — The  process  of  mastication  by  means  of- 
which  the  food  is  subjected  to  a  vigorous  mechanical  treat- 
ment, takes  place  in  the  mouth,  a  chamber  possessing  a 
relatively  unyielding  roof  but  movable  sides  and  floor.  The 
entrance  to  this  cavity  is  guarded  by  the  lips,  and  its  com- 
munication with  the  pharynx  by  vertically  placed  folds, 
called  the  pillars  of  the  fauces,  and  a  central  fleshy  curtain, 
termed  the  uvula.  Both  orifices  are  closed  by  muscular 
action,  the  aforesaid  parts  acting  in  the  manner  of  bi- 
lipped  valves.  The  roof  of  this  cavity  is  formed  by  the  hard 
and  soft  palates,  its  sides  by  the  cheeks,  and  its  floor  by  the 
tongue  and  adjoining  soft  parts,  held  in  the  solid  bony  frame 
of  the  mandible. 

The  muscles  taking  part  in  this  process  are  arranged  in 
such  a  way  that  the  lower  jaw  may  be  either  lowered  or 
raised,  moved  from  side  to  side,  or  protruded.  Its  relatively 
free  manner  of  movement  is  made  possible  by  the  fact 
that  the  condyle  of  its  ramus  articulates  with  the  temporal 
bone  by  means  of  a  double  condyloid  joint,  the  capsular 
ligament  of  which  is  rather  loose,  although  very  strong.  The 
raising  of  the  lower  jaw  is  effected  by  the  combined  contrac- 
tion of  the  temporal,  masseter,  and  internal  pterygoid 
muscles,  and  its  lowering  by  gravity  and  the  contraction  of 
the  digastric,  mylohyoid  and  geniohyoid  muscles.  When 
both  external  pterygoids  contract,  the  jaw  is  protruded, 
while  the  activation  of  the  internal  pterygoids  causes  it  to 


SALIVARY    DIGESTION  227 

recede.  The  contraction  of  only  one  set  of  these  antagonistic 
muscles  gives  rise  to  a  lateral  deviation  of  the  lower  jaw  in 
either  direction. 

The  maceration  of  the  material  ingested  is  accomplished 
chiefly  by  the  teeth  which  possess  a  somewhat  different 
shape  and  structure  in  accordance  with  the  character  of  the 
food  consumed  by  the  animal.  The  carnivora  do  not  masti- 
cate very  freely  and  rapidly  project  the  practically  unre- 
duced pieces  of  food  into  the  ossophagus  and  stomach, 
whereas  the  herbivora,  and  especially  the  ruminating  animals, 
break  them  up  into  small  fragments.  The  omnivora  occupy 
a  position  intermediate  between  these  two  groups  of  animals. 
Accordingly,  it  is  found  that  the  teeth  of  the  carnivora  are 
especially  adapted  to  catch  the  food,  while  those  of  the 
herbivora  present  the  characteristics  of  grinders.  It  may 
be  concluded,  therefore,  that  the  incisors  of  man  are  to  hold 
and  to  divide  the  food,  whereas  the  canines  break  it  up,  and 
the  bicuspids  and  molars  macerate  it. 

While  the  food  is  moved  about  in  the  mouth  and  finely 
subdivided,  it  is  thoroughly  mixed  with  the  secretions  of 
the  different  salivary  and  mucous  glands.  During  this  en- 
tire period  the  ptyalin  of  the  saliva  is  able  to  continue  its 
characteristic  action  upon  the  starches.  Eventually  the 
fragments  of  the  food  are  again  united  into  a  soft  and  rounded 
mass,  the  bolus,  which  is  then  projected  by  muscular  action 
into  the  stomach. 

The  Glands  of  the  Mouth. — The  lining  of  the  oral  cavity 
contains  many  mucous  glands  which  furnish  a  watery  medium 
in  which  considerable  amounts  of  mucin  are  suspended. 
This  secretion  merely  serves  the  purpose  of  a  lubricating 
fluid.  It  should  also  be  noted  that  this  cavity  gives  lodg- 
ment to  the  sublingual  and  faucial  tonsils,  the  former  being 
situated  below  the  tongue,  and  the  latter  between  the  pillars 
of  the  fauces.  These  masses  of  lymphoid  tissue  aid  in  the 
formation  of  small  white  corpuscles  which  are  transferred 
from  here  into  the  lymphatic  channels  of  the  neck  and  thence 
into  the  venous  bloodstream.  It  has  been  established  that 
these  organs  do  not  furnish  an  internal  secretion.  Inasmuch 
as  white  blood  corpuscles  are  greatly  needed  during  the 


228  NUTRITION 

early  years  of  our  life,  it  cannot  surprise  us  to  find  that  the 
tonsils,  together  with  the  oilier  lymphatic  glands,  are  usu- 
ally of  larger  size  in  children  than  in  adults.  A  general 
atrophy  of  these  organs  sets  in  about  the  twentieth  year. 
.A  third  organ  of  this  character  is  the  so-called  pharyngeal 
tonsil  or  adenoid  which  occupies  a  central  position  upon  the 
posterior  wall  of  the  pharynx.  Owing  chiefly  to  their  ex- 
posed position,  the  tubular  glands  of  these  organs  frequently 
become  infected  and  may  then  serve  as  channels  of  entrance 
for  various  bacteria  which  finally  reach  the  general  circula- 
tion through  the  cervical  lymphatics.  Under  these  circum- 
stances, as  well  as  when  these  organs  become  so  large  that 
they  seriously  interfere  with  the  passage  of  the  respiratory 
air,  their  removal  is  to  be  strongly  recommended. 

The  glands  secreting  the  saliva  are  known  as  the  parotid, 
submaxillary,  and  sublingual.  They  are  paired  organs. 
The  parotid  gland  occupies  the  space  in  front  of  the  ear, 
resting  with  its  flattened  inner  surface  upon  the  soft  parts 
covering  the  ramus  of  the  mandible.  The  membranous 
tube  through  which  its  secretion  is  conveyed  into  the  cavity 
of  the  mouth  is  known  as  Stenson's  duct.  Its  orifice  lies 
opposite  the  second  upper  molar  tooth,  where  it  is  recogniz- 
able as  a  blunt  prominence  or  papilla  upon  the  inner  surface 
of  the  cheek.  The  submaxillary  gland  is  situated  in  a  groove 
upon  the  inner  surface  of  the  lower  jaw,  between  this  bone  and 
the  tongue.  Its  duct  passes  forward  along  the  floor  of  the 
mouth  and  eventually  opens  next  to  the  frenulum  of  the 
tongue.  It  is  known  as  Wharton's  duct.  If  the  mouth  is 
rinsed  out  with  a  few  drops  of  diluted  vinegar,  drops  of  sub- 
maxillary saliva  will  be  seen  to  ooze  forth  from  this  orifice. 
The  sublingual  gland  lies  still  farther  forward  in  the  cleft 
between  the  lower  jaw  and  the  floor  of  the  mouth.  Its 
secretion  is  usually  collected  by  a  special  tube  which  is 
called  the  duct  of  Ravinus. 

The  Minute  Structure  of  the  Salivary  Glands. — The 
cells  of  the  different  salivary  glands  are  arranged  in  groups 
around  the  dilated  ends  of  the  finest  radicles  of  the  duct. 
Such  a  group  of  chief  cells  constitutes  an  acinus.  In  the 
case  of  the  submaxillary  gland,  the  chief  cells  are  limited 


SALIVARY    DIGESTION  229 

externally  by  a  second  type  of  cell  which  possesses  a  crescent 
shape  and  is  known  as  a  demilune  cell.  It  should  be  noted 
first  of  all  that  the  quality  of  the  saliva  closely  corresponds 
to  the  general  character  of  the  gland,  the  most  watery 
or  serous  type  of  secretion  being  furnished  by  the  parotids, 
and  the  thickest  by  the  sublinguals.  The  submaxillary 
glands  produce  an  intermediate  quality  of  saliva.  For  this 
reason,  the  parotids  are  usually  designated  as  albuminous 
glands  and  the  submaxillary  and  sublinguals,  as  mucous  glands. 
These  differences,  however,  are  not  only  recognizable  in 
variations  in  the  stickiness  or  viscidity  of  the  saliva,  but 
also  in  variations  in  its  content  in  solids.  Obviously,  the 
viscidity  of  a  secretion  depends  upon  its  content  in  mucin, 
while  its  concentration  is  determined  by  its  solids,  chief 
among  which  are  inorganic  salts  and  albumin.  The  varying 
structure  of  these  organs,  therefore,  leads  us  to  infer  that 
each  gland  furnishes  its  own  peculiar  type  of  secretion,  and 
that  a  thorough  mixture  of  them  is  effected  only  after  they 
have  been  poured  into  the  cavity  of  the  mouth. 

It  is  also  of  interest  to  note  that  these  cells  present  certain 
very  characteristic  changes  after  they  have  been  made  to 
secrete  for  some  time.  Thus,  it  may  easily  be  observed 
that  the  cells  of  a  resting  gland  are  large  and  densely  loaded 
with  granular  material  from  which  the  solids  of  the  saliva 
are  derived.  Their  nuclei  are  irregular  in  outline  and  occupy 
a  position  near  the  capillary  sides  of  the  cells.  When  a  gland 
is  made  to  secrete  excessively  by  the  stimulation  of  its  nerve, 
its  cells  grow  smaller  in  size,  while  the  nuclei  become  rounded 
and  move  into  a  position  near  the  center  of  the  cytoplasm. 
The  dark  granules  which  were  formerly  so  widely  distributed 
through  the  cell,  are  now  few  in  number  and  lie  chiefly  in 
the  zone  nearest  the  duct.  These  changes  lead  us  to  infer 
that  many  of  these  granules  have  been  transferred  into  the 
duct  to  become  constituents  of  the  secretion.  Such  struc- 
tural alterations  are  also  discernable  in  the  cells  of  the  pan- 
creas and  intestinal  glands;  in  fact,  they  are  displayed  by  all 
glands  during  their  periods  of  activity. 

The  Innervation  of  the  Salivary  Glands. — The  secretion  of 
saliva  belongs  in  the  class  of  the  involuntary  or  vegetative 


230 


NUTRITION 


functions.  This  statement  implies  that  it  is  not  under  the 
control  of  the  will  and  follows,  therefore,  only  upon  reflex 
stimulation.  The  ganglion  cells  controlling  this  action  are 
situated  in  the  medulla  oblongata,  and  form  a  special  center 
which  is  known  as  the  salivary  center.  With  the  help  of 
diverse  afferent  paths  the  activity  of  the  latter  may  be 
varied  at  any  time  in  accordance  with  the  character  of  the 
impulses  received.  Its  motor  discharges  are  relayed  to  the 
glands  by  means  of  two  paths,  one  of  which  pursues  a  direct 


FIG.  93. — Acini  of  the  submaxillary  gland  during  rest  (R)  and  activity 
(A).     The   dark  outer   cells  represent  the   demilune   cells. 


course  through  the  channels  of  the  cranial  nerves,  and  the 
other  an  indirect  course  through  the  thoracic  and  cervical 
sympathetic  system.  In  the  latter  case,  the  path  of  the 
motor  fibers  has  not  been  ascertained  as  yet  with  certainty. 
It  is  also  of  interest  to  note  that  the  excitation  of  these 
nerves  gives  rise  to.  the  formation  of  two  very  different  types 
of  saliva.  Inasmuch  as  the  same  results  are  obtained  with 
all  three  salivary  glands,  it  may  suffice  to  illustrate  this 
particular  point  by  briefly  describing  the  changes  which  this, 
procedure  induces  in  the  submaxillary  gland.  This  organ 
receives  its  direct  supply  of  cerebral  fibers  through  a  nerve,  to 
which  the  name  of  chorda  tympani  has  been  given,  because  it 
traverses,  during  a  part  of  its  course,  the  cavity  of  the  middle 
ear  or  tympanum.  Its  sympathetic  innervation  is  derived 
from  the  superior  cervical  ganglion  which  forms  the  upper- 


SALIVAKY    DIGESTION  231 

most  station  of  this  system.  Distally  to  this  point,  these 
nerve  fibers  follow  the  highway  of  the  carotid  artery  to  the 
gland. 

When  the  chorda  tympani  is  stimulated  the  following 
changes  may  be  noted:  (a)  the  bloodvessels  of  this  gland 
dilate,  thereby  augmenting  its  volume  and  temperature; 
(6)  the  flow  of  saliva  is  greatly  increased;  and  (c)  the  saliva 
assumes  a  very  watery  consistency.  Contrariwise,  the 
excitation  of  the  sympathetic  fibers  produces  a  constriction 
of  the  bloodvessels  of  this  gland,  and  gives  rise  to  the  secre- 
tion of  a  small  quantity  of  very  thick  saliva.  It  appears, 
therefore,  that  the  cells  of  this  gland  are  minute  laboratories 
which  possess  a  definite  vital  activity  which  in  a  large  meas- 
ure is  quite  independent  of  the  bloodsupply.  We  shall  see 
later  that  this  statement  may  rightly  be  made  regarding  all 
secretory  cells. 

Under  normal  conditions  the  nervous  mechanism  controll- 
ing the  secretion  of  saliva  is  activated  by  afferent  impulses 
which  originate  either  outside  or  inside  the  oral  cavity.  It  is 
a  matter  of  common  experience  that  the  smelling  or  seeing 
of  food  frequently  suffices  to  evoke  a  copious  production  of 
this  secretion.  These  exherent  stimuli  invariably  give  rise 
to  a  quick  reaction  and  produce  what  is  known  as  the  psychic 
secretion  of  saliva.  After  the  food  has  been  placed  in  the 
mouth  it  stimulates  the  receptors  of  the  mucous  membrane 
of  this  cavity  in  a  mechanical  as  well  as  chemical  manner. 
These  inherent  stimuli  produce  a  secretion  which  is  sustained 
for  a  much  longer  period  of  time  and  takes  the  place  of  the 
quick  but  brief  psychic  flow.  Obviously,  the  sense-organs 
upon  which  the  food  exerts  its  direct  action  are  the  taste- 
buds  of  the  tongue,  fauces,  and  cheeks. 

The  Character  and  Action  of  Saliva. — Saliva  consists  of  a 
very  large  amount  of  water,  holding  in  solution  a  small 
quantity  of  protein  material,  mucin,  and  inorganic  salts. 
Its  reaction  is  neutral  or  slightly  alkaline.  Its  active 
principle  is  the  enzyme  ptyalin  which  possesses  a  selective 
action  upon  starch,  changing  it  through  several  intermediary 
stages  into  maltose.  But  maltose  as  such  cannot  be  absorbed 
until  it  has  been  further  reduced  into  simple  sugar.  This 


232  NUTRITION 

additional  change  is  accomplished  by  the  enzyme  amylopsin 
of  the  pancreatic  juice  and  a  similar  one  contained  in  the 
intestinal  juice.  It  will  be  seen,  therefore,  that  any  portion 
of  the  starch  ingested  that  has  escaped  the  action  of  the 
ptyalin,  is  not  lost  to  the  body. 

The  digestive  power  of  the  saliva,  however,  is  relatively 
slight  as  may  be  gathered  from  the  fact  that  most  animals  do 
not  masticate  very  thoroughly,  and  that  ptyalin  is  absent  in 
many  of  them.  Thus,  while  it  may  be  said  that  this  secre- 
tion possesses  the  aforesaid  chemical  action,  its  principal 
use  seems  to  be  rather  a  mechanical  one,  because  it  moistens 
the  food  and  renders  the  mucous  surfaces  more  slippery. 


CHAPTER  XXIII 
GASTRIC  DIGESTION 

The  Act  of  Swallowing  or  Deglutition. — It  has  been  stated 
above  that  the  end-product  of  mastication  is  the  bolus,  a 
rounded  mass  of  food  thoroughly  moistened  with  saliva. 
In  this  form  the  food  is  projected  into  the  stomach,  where  it 
is  again  subjected  to  a  mechanical  and  chemical  reduction 
until  it  has  been  converted  into  a  liquid  of  high  acidity, 
termed  the  chyme.  The  act  of  swallowing  may  be  divided 
into  three  stages.  The  first  of  these  is  completed  when  the 
bolus  reaches  the  aperture  of  the  fauces;  the  second  when  it 
enters  the  upper  orifice  of  the  oesophagus;  and  the  third, 
when  it  has  passed  the  cardiac  sphincter  of  the  stomach. 

During  the  first  stage  the  oral  cavity  is  gradually  obliter- 
ated from  before  backward,  so  that  the  bolus  is  forced  into 
the  pharynx,  i.e.,  in  the  direction  of  least  resistance.  This 
act  is  accomplished  volitionally,  and  although  several 
striated  muscles  take  part  in  it,  the  most  important  are  those 
moving  the  tongue.  This  organ  is  raised  progressively, 
beginning  with  its  tip,  until  its  upper  surface  has  been 
brought  in  absolute  contact  with  the  hard  palate.  The 
bolus  is  therebv  forced  to  escape  through  the  aperture  of 
the  fauces.  Having  reached  the  pharynx,  the  bolus  is 
grasped  by  the  sphincters  of  its  upper  and  middle  segments 
and  directed  downward  into  the  orifice  of  the  oesophagus. 
At  this  time  the  posterior  wall  of  this  cavity  is  brought 
forward  in  close  contact  with  the  pillars  of  the  fauces  and 
the  uvula.  The  bolus  is  now  propelled  through  the  oesopha- 
gus by  peristaltic  action.  As  has  been  stated  above,  a 
peristaltic  wave  consists  essentially  of  a  zone  of  constriction 
which  is  preceded  by  a  zone  of  relaxation.  The  bolus  occu- 
pies the  relaxed  area  ahead  of  the  constriction. 

Semi-solid  material  requires  about  six  seconds  for  its 
passage  from  the  mouth  to  the  stomach.  This  interval, 

233 


234  NUTRITION 

however,  is  largely  taken  up  by  its  journey  through  the 
oesophagus,  because  this  membranous  tube  communicates 
with  the  stomach  by  an  orifice  which  is  kept  closed  by  a 
tonically  contracted  band  of  circular  muscle  fibers.  As  in 
other  localities,  this  sphincter  must  first  be  opened  by  certain 
reflex  stimuli  which,  in  this  case,  appear  to  be  wholly  me- 
chanical in  nature.  Fluids,  on  the  other  hand,  do  not 
elicit  complete  peristaltic  waves,  and  are  quickly  projected 
into  the  lower  segment  of  the  oesophagus. 

It  is  a  matter  of  common  experience  that  the  successive 
acts  of  swallowing  must  be  separated  from  one  another  by 
a  definite  interval,  so  that  a  sufficient  time  may  be  allowed 
the  food  to  traverse  the  sphincter.  If  these  intervening 
periods  are  made  shorter  than  one  second,  the  different 
motor  reactions  cannot  be  executed  with  sufficient  precision 
to  prevent  the  entrance  of  food  into  the  trachea  or  to  avoid 
the  accumulation  of  an  excessive  amount  of  food  above  the 
sphincter.  Such  a  stagnation  of  food  usually  produces 
severe  pains  in  the  region  of  the  cardia  of  the  stomach 
which  radiate  upward  along  the  oesophagus. 

Salivary  digestion  is  usually  continued  for  some  time 
after  the  bolus  has  entered  the  cardiac  end  of  the  stomach. 
Obviously,  since  the  ptyalin  retains  its  digestive  power  only 
in  a  neutral  or  slightly  alkaline  medium,  its  action  upon  the 
starches  must  cease  as  soon  as  all  parts  of  the  bolus  have  been 
thoroughly  moistened  with  the  acid  gastric  juice.  This 
period  during  which  it  continues  its  action  is  about  15  or  20 
minutes  in  duration. 

The  Stomach. — In  man  the  stomach  appears  as  a  single 
saccular  enlargement  of  the  alimentary  canal.  It  is  situated 
in  the  left  hypochondriac  and  epigastric  regions  of  the 
abdominal  cavity,  and  extends  downward  to  a  horizontal  line 
drawn  about  2  cm.  above  the  umbilicus.  The  space  occupied 
by  it  varies  with  the  state  of  its  distention;  however,  even' 
the  empty  organ  is  not  fully  collapsed,  but  contains  a  certain 
amount  of  frothy  material.  Its  left  extremity  is  very  spaci- 
ous, extending  about  5  cm.  to  the  left  of  the  cesophageal 
orifice,  while  its  right  extremity  is  funnel-shaped,  terminating 
finally  somewhat  to  the  right  of  the  linea  alba  in  the  region 


GASTRIC    DIGESTION 


235 


of  the  xiphoid  cartilage.  When  highly  distended  with  food, 
its  right  pole  may  come  to  lie  behind  the  end  of  the  cartilage 
of  the  eighth  rib.  Its  left  portion  is  termed  the  cardia  and  its 
right  portion,  the  pylorus.  The  large  cul-de-sac  below  the 
orifice  of  the  oesophagus  is  called  the  fundus.  The  stomach 
as  a  whole  occupies  a  transverse  position,  its  long  convex 


FIG.  94. — Stomach  and  beginning  portion  of  intestine. 
Oes.,  oesophagus;   C,  cardia;  F,  fundus;  P,  pylorus;  L  and  G,  lesser  and  greater 
curvatures;  D,   common  bile  duct;   HD,  hepatic  duct;   CD,  cystic  duct;  GB,  gall 
bladder;  Li,  liver;  Do,  duodenum;  Pa,  pancreas;  and  W,  duct  of  Wirsung. 

border  or  greater  curvature  being  turned  forward  and  doVn- 
ward,  while  its  short  concave  border  or  lesser  curvature  is 
directed  backward  and  upward. 

The  wall  of  this  organ  consists  of  a  mucous  lining,  a 
middle  layer  of  muscle  tissue,  and  an  external  investing 
membrane  of  peritoneum.  Its  mucosa  contains  numerous 
tubular  glands  which  secrete  the  gastric  juice.  Between 
these  we  also  find  a  number  of  smaller  glands  which  furnish 
a  mucous  fluid  for  purposes  of  lubrication.  Its  muscular 


236 


NUTRITION 


coat  is  formed  by  smooth  muscle  cells  which  are  arranged 
either  transversely  or  longitudinally  to  the  long  axis  of  its 
cavity.  Upon  the  cardia  are  also  found  a  certain  number  of 
oblique  fibers  which  are  continuous  with  the  circular  fibers  of 
the  ossophagus  and  terminate  in  its  inner  circular  layer. 

The  layer  of  circular  muscle  tissue  is  especially  conspicuous 
at  the  point  where  the  oesophagus  joins  the  cardia,  as  well  as 
at  the  junction  between  the  pylorus  and  the  upper  segment 
of  the  small  intestine  or  duodenum.  Two  distinct  muscular 


CS 


FIG.  95. — Diagrammatic  representation  of  the  stomach.  C,  cardiac 
end;  F,  fundus;  P,  pylorus;  D,  duodenum;  CS,  cardiac  sphincter;  SA, 
sphincter  antri  pylori;  PS,  pyloric  sphincter;  V,  valvulae  conniventes. 


stops  are  formed  at  these  two  points  which  are  known 
respectively  as  the  cardiac  and  pyloric  sphincters  of  the 
stomach.  In  fact,  the  human  stomach  frequently  exhibits  a 
third  band  of  circular  fibers  at  the  junction  between  the 
pylorus  and  fundus.  This  ring  of  muscle  tissue  is  commonly 
designated  as  the  sphincter  of  the  pyloric  vestibule  or  sphincter 
antri  pylori.  This  central  stop  usually  remains  relaxed, 
although  it  may  assume  a  constricted  condition  at  any  time 
in  consequence  of  various  excitations  of  the  gastric  mucosa. 
Such  a  stimulation  is  prone  to  arise  when  the  mucosa  of  the 
pylorus  has  become  the  seat  of  an  ulcer,  or  when  the  gastric 


GASTRIC    DIGESTION  237 

juice  is  excessively  acid.  In  transillumination  this  organ 
then  possesses  the  shape  of  an  hour-glass.  In  this  connection 
brief  reference  should  be  made  to  the  stomach  of  the  ruminat- 
ing animals  which  consists  of  four  consecutive  compartments. 
It  is  also  of  interest  to  note  that  the  human  stomach  may 
appear  in  the  form  of  two  separate  organs,  arranged  in 
series,  one  occupying  a  horizontal  and  the  other  a  vertical 
position. 

The  Gastric  Juice. — The  character  of  the  gastric  juice  has 
been  well  known  since  about  the  year  1845,  when  Alexis 
St.  Martin,  a  Canadian  hunter,  met  with  a  peculiar  accident 
which  extensively  lacerated  his  abdominal  wall  and  adjoining 
portion  of  the  stomach.  In  healing,  a  fistulous  communica- 
tion was  established  between  the  outside  and  the  cavity  of 
this  organ,  through  which  food  could  be  introduced  and 
again  removed  at  a  later  hour  to  see  what  changes  had  been 
produced  therein  by  the  gastric  juice.  A  number  of  similar 
cases  have  been  reported  in  more  recent  years,  because  all 
complete  obstructions  of  the  oesophagus  by  growths  or  in 
consequence  of  erosions  by  corrosive  liquids,  are  now  relieved 
by  establishing  a  direct  communication  through  the  walls 
of  the  abdomen  and  stomach.  Gastric  juice  may  also  be 
obtained  by  aspirating  or  siphoning  it  through  a  long  tube 
of  rubber  inserted  through  the  oesophagus. 

When  collected  from  a  fasting  person,  the  gastric  juice  is 
quite  clear,  odorless,  acid  in  reaction,  and  sour  to  the  taste. 
It  is  secreted  constantly,  although  in  small  amounts.  The 
ingestion  of  food,  however,  calls  forth  a  much  more  copious 
flow,  amounting  in  dogs  to  as  much  as  one  liter  in  the  course 
of  three  hours.  Human  beings  produce  about  700  c.c.  of  this 
secretion  during  a  moderate  meal.  It  contains  three  active 
agents:  namely,  hydrochloric  acid,  pepsin,  and  rennin. 

The  hydrochloric  acid  is  derived  from  the  chlorids  of  the 
blood.  Although  present  only  in  amounts  sufficient  to 
raise  the  acidity  of  the  secretion  to  0.2  per  cent.,  it  destroys 
many  of  the  micro-organisms  swallowed  with  the  food,  and 
aids  in  the  closure  and  opening  of  the  cardiac  and  pyloric 
sphincters.  Furthermore,  on  being  ejected  into  the  duo- 
denum, it  leads  to  the  liberation  of  the  hormone  "secretin" 


238  NUTRITION 

from  the  cells  lining  the  upper  intestinal  tract.  We  shall 
see  later  that  this  agent  stimulates  the  flow  of  the  pancreatic 
juice,  intestinal  juice,  and  bile.  The  direct  influence  of  the 
hydrochloric  acid  upon  the  foods  consists  in  its  establishing 
a  suitable  medium  for  the  powerful  enzyme  pepsin  to  act  in. 
It  also  aids  in  the  erosion  and  destruction  of  the  cellulose  and 
connective  tissue  investments  of  the  foods. 

Pepsin  is  a  proteolytic  enzyme,  and  converts  the  proteins 
through  several  intermediary  stages  into  peptones.  It  should 
be  mentioned  at  this  time  that  the  final  products  of  protein 
digestion  are  the  amino-acids.  The  gastric  juice,  however, 
does  not  carry  their  cleavage  quite  so  far  as  that,  but  leaves 
their  ultimate  reduction  to  the  corresponding  enzymes  of 
the  pancreatic  and  intestinal  juices. 

Rennin  is  a  special  proteolytic  agent  set  aside  for  the 
simplification  of  the  protein  of  milk,  which  is  known  as 
casein.  While  the  relationship  between  this  enzyme  and  the 
pepsin  has  not  been  definitely  established  as  yet,  it  is  obvious 
that  it  possesses  a  very  characteristic  action,  consisting  in 
the  formation  of  the  curd.  During  this  change  in  the  con- 
sistency of  the  milk,  the  protein  is  transferred  from  its 
soluble  form  or  caseinogen  into  its  insoluble  form  or  casein. 
The  casein  is  then  reduced  by  the  pepsin  into  the  correspond- 
ing peptone.  This  reaction  serves  as  the  basis  of  cheese- 
making,  the  " rennet"  employed  to  curdle  the  milk  being 
obtained  by  scraping  and  extracting  the  mucous  lining  of  the 
stomach  of  the  calf. 

The  gastric  juice  does  not  possess  a  clearly  recognizable 
action  upon  the  fats.  Evidently,  pure  fat,  as  is  found  in 
butter  and  lard,  is  not  attacked,  although  emulsified  fats,  like 
cream  and  oil,  may  be  reduced  to  some  extent  into  glycerin 
and  fatty  acids.  The  enzyme  responsible  for  this  action  is 
called  gastric  lipase.  The  fat  of  meat  is  imbedded  in  a  pro^ 
tein  framework  which  is  destroyed  by  the  pepsin  in  the 
presence  of  hydrochloric  acid.  The  carbohydrates  are  not 
chemically  altered  by  the  gastric  juice,  although  their 
capsular  investments  may  be  softened  and  eroded. 

The  Gastric  Glands. — The  mucous  membrane  of  the 
stomach  contains  a  very  large  number  of  tubular  glands 


GASTRIC    DIGESTION 


239 


which  present  the  following  characteristics:  Their  lumen  is 
lined  with  large  cubical  and  slightly  granular  cells  which  are 
known  as  chief  cells.  Outside  these  lie  at  irregular  intervals 
numerous  oval  cells  which  are  designated  as  parietal  cells. 


FIG.  96. — Diagrammatic    representation    of    a    fundic    gland.     C,    chief 
cells;  P,  parietal  cells;  D,  duct  of  gland;  N,  neck  of  gland. 

Communication  is  formed  between  the  latter  and  the  lumen 
of  the  duct  by  means  of  delicate  tubules  which  traverse  the 
intercellular  substance  between  the  chief  cells.  Each  gland 
is  connected  with  the  cavity  of  the  stomach  by  means  of  a 
pore-like  orifice. 

It  should  be  noted  that  the  glands  situated  in  the  pyloric 
end  of  the  stomach,  are  devoid  of  parietal  cells,  and  yield  a 


240  NUTRITION 

secretion  somewhat  different  from  that  furnished  by  the 
glands  in  the  fundic  portion  of  this  organ.  This  difference 
pertains  chiefly  to  its  content  in  hydrochloric  acid.  Thus, 
if  the  stomach  of  a  mammal  is  divided  into  two  compart- 
ments, it  will  be  found  that  the  secretion  from  its  pyloric 
pocket  is  alkaline,  while  that  from  its  fundic  portion  is  acid. 
Now,  since  the  glands  of  the  pylorus  do  not  embrace  parietal 
cells,  while  those  of  the  fundus  do,  it  may  justly  be  concluded 
that  these  cellular  units  furnish  the  acid,  while  the  chief  cells 
produce  the  pepsin.  This  conclusion  has  recently  been 
fully  substantiated  by  the  process  of  vital  staining. 

Inasmuch  as  pepsin  in  the  presence  of  hydrochloric  acid 
unfolds  such  powerful  proteolytic  properties,  it  may  seem 
strange  that  it  does  not  attack  the  wall  of  the  stomach. 
It  is  a  well  known  fact  that  erosions  and  ulcers  of  the  gastric 
mucosa  result  only  when  the  circulation  has  been  interfered 
with  sufficiently  to  evoke  a  disturbance  in  the  oxidative 
processes  of  the  lining  cells.  Furthermore,  the  pepsin  is 
contained  in  the  chief  cells  in  the  form  of  its  inactive  mother- 
substance  pepsinogen,  and  develops  its  digestive  power  only 
after  it  has  been  cast  into  the  acid  gastric  juice.  Lastly,  it  is 
entirely  probable  that  the  pepsin  is  rendered  inert  when 
brought  in  contact  with  the  lining  cells.  It  has  also  been 
stated  that  the  secretion  of  the  mucous  glands  serves  as  a 
protective  covering  for  these  cells,  but  this  contention  cannot 
be  substantiated,  because  a  highly  acid  stomach  usually 
contains  only  a  very  small  amount  of  mucus  and  does  not, 
as  a  rule,  present  erosions  of  its  lining.  Contrariwise,  it  is 
frequently  observed  that  a  low  acid  content  favors  the  pro- 
duction of  mucus,  and  that  ulcerations  of  the  gastric  wall  are 
quite  common  at  this  time. 

The  Secretion  of  Gastric  Juices. — It  has  previously  been 
noted  that  small  quantities  of  gastric  juice  are  secreted 
even  during  the  interims  between  meals.  Moreover,  the 
intake  of  food  greatly  augments  its  flow.  The  stimuli 
giving  rise  to  this  increased  secretion,  may  be  classified  as 
exherent  and  inherent.  Thus,  it  has  been  proven  that  the 
gastric  glands  are  activated  some  time  before  the  food  has 
actually  entered  the  stomach.  Certain  psychic  stimuli  are 


GASTRIC    DIGESTION  241 

responsible  for  their  activation.  Consequently,  the  condi- 
tions met  with  here  are  very  similar  to  those  previously 
noted  in  the  case  of  salivary  secretion,  because  the  flow  of  this 
digestive  fluid  follows  very  quickly  upon  the  mere  seeing  and 
smelling  of  food.  In  both  instances,  however,  this  early 
psychic  secretion  is  not  sustained  for  any  considerable  length 
of  time.  After  the  food  has  entered  the  mouth  and  has 
been  transferred  into  the  stomach,  it  exerts  a  mechanical  as 
well  as  chemical  influence  upon  the  lining  membrane  of  this 
organ.  This  gives  rise  to  a  much  more  prolonged  outpouring 
of  gastric  juice. 

The  mechanical  stimulation  consists  in  impacts  of  the 
semi-solid  constituents  of  the  food  upon  the  gastric  mucosa. 
The  chemical  agents  present  themselves  in  two  forms: 
namely,  as  peculiar  admixtures  of  the  food  which  exert  a 
stimulating  influence  upon  the  digestive  processes,  and  as 
substances  which  arise  in  the  stomach  itself  and  excite  its 
glands  directly.  The  former  are  known  as  vitamines,  and  the 
latter  as  secretagogues.  Gastrin  is  the  name  applied  to  the 
secretagogue  liberated  in  this  organ. 

Very  little  is  known  regarding  the  chemical  nature  of  the 
vitamines,  although  their  presence  in  food  has  been  estab- 
lished beyond  a  doubt.  Such  diseases  as  scurvy,  beri-beri, 
pellagra,  and  rickets  have  their  origin  in  the  continuous 
ingestion  of  vitamine-free  substances.  Scurvy,  for  example, 
used  to  be  prevalent  upon  sailing  vessels  when  fresh  meat, 
vegetables  and  fruits  were  unobtainable,  and  when  fish 
formed  the  chief  article  of  diet.  In  order  to  prevent  this 
"deficiency  disease,"  lime  and  lemon-juice  were  administered 
at  intervals,  because  these  articles  contain  the  stimulating 
agents  of  citric  acid  and  malic  acid.  Furthermore,  statistics 
clearly  prove  that  beri-beri  has  greatly  increased  in  the 
East  since  the  introduction  of  modern  processes  of  milling 
which  relieve  the  kernels  of  the  grain  of  their  capsules. 
Grain  prepared  with  the  capsules  intact  does  not  give  rise 
to  this  disease.  Polished  rice  is  equally  injurious  when 
employed  as  an  exclusive  diet  for  long  periods  of  time.  The 
same  may  be  said  regarding  pasteurized  milk  when  fed  to 
infants  to  the  exclusion  of  other  suitable  foods. 

16 


242  NUTRITION 

It  is  very  fortunate,  however,  that  these  metabolic  dis- 
orders may  be  remedied  without  great  difficulty  by  the 
administration  of  substances,  known  to  be  rich  in  vitamines. 
This  point  may  be  more  fully  illustrated  by  referring  to  those 
infants  who  have  been  rendered  scorbutic  by  the  exclusive 
feeding  of  pasteurized  milk.  When  improperly  executed, 
this  manner  of  " purifying"  the  milk  removes  from  it  a 
certain  constituent  which  is  absolutely  essential  to  the  meta- 
bolic requirements  of  the  infant.  This  constituent  belongs 
to  the  group  of  the  vitamines,  but  its  chemical  nature  is 
wholly  unknown.  The  alarming  symptoms  developed  during 
this  disease,  may  be  remedied  within  a  day  or  two  by  the 
feeding  of  orange  juice  and  white  of  egg. 

The  Movements  of  the  Stomach. — It  is  to  be  noted  espe- 
cially that  the  fundic  portion  of  the  stomach  remains  relatively 
quiescent,  and  serves  merely  as  a  reservoir  for  the  pylorus. 
The  latter  is  very  active  especially  after  meals.  It  then 
shows  typical  peristaltic  movements,  consisting  of  waves  of 
constriction  which  are  preceded  by  waves  of  relaxation. 
These  movements  begin  at  the  junction  between  the  pylorus 
and  fundus,  and  slowly  progress  from  here  toward  the 
sphincter  of  the  pylorus.  At  the  height  of  gastric  digestion 
as  many  as  three  of  these  waves  may  be  observed  at  any  one 
time. 

Their  character  and  effect  may  best  be  observed  by  means 
of  the  Rontgen-rays  after  the  ingestion  of  food  containing  a 
certain  amount  of  subnitrate  of  bismuth.  This  salt  does 
not  permit  these  rays  to  pass,  and  causes  the  stomach  to  be 
outlined  in  the  form  of  a  shadow  upon  the  barium  screen. 
It  has  been  stated  above  that  the  cardiac  and  fundic  portions 
of  this  organ  merely  serve  as  a  reservoir  which  holds  the  food 
in  readiness  until  acted  upon  by  the  pylorus.  Their  con- 
tents are  arranged  in  such  a  way  that  the  material  ingested 
most  recently  occupies  a  central  position  below  the  ceso- 
phageal  orifice,  while  the  older  food  is  gradually  forced  down- 
ward and  outward  toward  the  line  of  junction  between  the 
fundus  and  pylorus.  Every  new  wave  of  peristalsis  develop- 
ing in  this  region  separates  a  relatively  small  mass  of  material 
from  that  contained  in  the  fundus  and  moves  it  toward 


GASTRIC    DIGESTION 


243 


the  pyloric  sphincter.  In  its  downward  journey  it  is  gradu- 
ally broken  up  into  smaller  pieces  which,  however,  are  not 
allowed  to  escape  directly  into  the  intestine  but  are  again 
diverted  toward  the  fundus,  occupying  at  this  time  a  posi- 
tion close  to  the  gastric  wall.  These  whirlpool  movements 
of  the  food  serve  the  purpose  of  reducing  it  in  a  mechanical 
way  and  thoroughly  moistening  it  with  the  gastric  juice. 


Fia.  97. — Shadows  of  the  human  stomach  obtained  with  the  aid  of  the 
Rontgen  rays  15  minutes,  1  hour,  and  4  hours  after  the  ingestion  of  the 
bismuth  meal. 

The  Evacuation  of  the  Stomach  Contents. — The  time 
during  which  the  food  is  retained  in  the  stomach,  depends 
upon  its  quantity  and  quality.  Water  and  carbohydrates 
usually  enter  the  duodenum  within  15  to  30  minutes  after 
their  ingestion,  whereas  proteins  and  fats  may  require 
several  hours  for  their  complete  reduction.  This  is  also 
true  of  a  mixed  meal  of  ordinary  bulk,  because  the  fats 
always  serve  as  retarding  agents.  At  all  events,  a  stomach 


244  NUTRITION 

which  does  not  empty  itself  completely  within  4  or  5  hours, 
is  not  in  a  proper  condition  of  tonus.  It  should  be  remem- 
bered, however,  that  this  organ  may  also  be  prevented  from 
emptying  itself  in  the  time  specified  by  an  obstruction  at 
the  pyloric  orifice,  such  as  may  be  produced  by  tumors  and 
ulcers. 

The  ultimate  purpose  of  gastric  digestion  is  the  formation 
of  the  chyme,  a  liquid  of  high  acidity  and  containing  practi- 
cally no  solid  material.  In  this  form  the  food  is  transferred 
into  the  duodenum.  This  process  of  evacuating  the  gastric 
contents  embodies  two  distinct  motor  reactions:  namely, 
certain  modified  contractions  of  the  wall  of  the  stomach  and 
the  relaxation  of  the  sphincter  of  the  pylorus.  The  cardiac 
.and  fundic  portions  of  the  stomach  are  then  raised  by  the 
contraction  of  the  oblique  muscle  fibers,  while  the  pylorus 
is  allowed  to  assume  a  dependent  position.  The  peristaltic 
movements  sweeping  over  the  pylorus,  now  force  the  chyme 
through  the  widely  opened  pyloric  orifice  into  the  upper 
portion  of  the  duodenum. 


CHAPTER  XXIV 
INTESTINAL  DIGESTION 

The  Pancreas  and  Its  Secretion. — At  a  distance  of  about 
8  cm.  below  the  pyloric  sphincter,  the  duodenum  receives 
two  ducts,  one  from  the  pancreas  and  one  from  the  liver. 
The  former  is  known  as  the  duct  of  Wirsung  and  the  latter 


FIG.  98. — Ventral  view  of  the  duodenum,  pancreas  and  its  neighboring 
organs.      (Radasch.) 

as  the  common  bile  duct.  Most  generally,  however,  these 
membranous  tubes  are  united  into  a  single  one  before  they 
actually  pierce  the  wall  of  the  duodenum.  The  pancreas  is 
a  tubulo-racemose  gland,  possessing  a  band-like  shape.  It 
consists  of  three  portions:  namely,  a  head,  body,  and 

245 


246  NUTRITION 

tail.  Its  body  attaches  itself  closely  to  the  duodenum, 
whereas  its  extremities  lie  free  in  the  mesentery.  The  sub- 
stance of  this  organ  is  made  up  of  many  lobules,  each  of  which 
embraces  a  considerable  number  of  acini,  i.e.,  groups  of  true 
secreting  cells  arranged  around  the  alveoli  of  excretory  ducts. 
These  cells  furnish  the  pancreatic  juice.  In  addition,  the 
pancreas  embraces  a  number  of  cells  which  are  not -directly 
concerned  with  the  formation  of  pancreatic  juice,  but  furnish 
an  internal  secretion  having  to  do  with  the  metabolism  of  the 
carbohydrates.  These  cells  are  situated  in  the  spaces  between 
the  different  acini,  forming  here  well  denned  structures  which 
are  called  the  islands  of  Langerhans.  It  will  be  shown  later 
that  the  destruction  of  the  latter  gives  rise  to  a  complex  of 
very  disturbing  symptoms,  constituting  the  disease  of  diabetes 
mellitus. 

The  Character  and  Action  of  the  Pancreatic  Juice. — When 
collected  directly  from  the  duct  of  Wirsung,  this  secretion 
appears  as  a  clear,  watery  fluid,  containing  considerable 
amounts  of  phosphates  and  carbonates,  and  especially  those 
of  sodium.  Because  of  the  presence  of  these  salts  it  possesses 
an  alkaline  reaction.  It  also  contains  an  appreciable  amount 
of  protein  in  solution,  and  three  powerful  enzymes  which 
are  usually  termed  trypsin,  amylopsin,  and  steapsin. 

Trypsin,  the  most  important  of  the  pancreatic  enzymes, 
is  proteolytic  in  nature.  It  is  retained  within  the  cells  of 
this  gland  in  the  form  of  trypsinogen,  an  inactive  constituent 
which,  however,  acquires  its  powerful  digestive  action  as 
soon  as  it  has  been  mixed  with  the  general  intestinal  juice. 
This  transformation  is  accomplished  by  the  enterokinase 
of  the  duodenal  juice  which  is  poured  into  the  intestinal 
canal  by  the  glands  of  Brunner,  situated  directly  below  the 
pyloric  orifice.  While  the  trypsin  continues  the  action 
of  the  pepsin,  it  differs  from  the  latter  in  two  particulars,  i.e., 
it  acts  only  in  an  alkaline  medium  and  advances  the  cleavage" 
of  the  protein  molecules  beyond  the  stage  of  peptones  into 
that  of  the  amino-acids.  These  end-products  are  of  peculiar 
interest,  because  they  form  protein  " building  stones" 
which  are  subsequently  employed  by  the  cells  of  the  tissues 
in  the  reconstruction  of  their  substance. 


INTESTINAL   DIGESTION  247 

Amylopsin  bears  a  close  resemblance  to  the  ptyalin  of  the 
saliva,  because  it  converts  starch  into  maltose  (C^H^On), 
and  is  able  to  take  up  the  work  of  this  amylolytic  agent  at 
the  dextrin  stage,  reducing  the  complex  sugars  into  their 
simplest  forms.  It  will  be  seen,  therefore,  that  those  com- 
plex carbohydrates  which  have  escaped  salivary  digestion, 
are  here  confronted  by  a  much  more  energetic  amylase  and 
diastase  than  the  ptyalin. 

Steapsin  is  a  lipase,  an  enzyme  specifically  adapted  to 
digest  fat.  Under  its  influence  the  ordinary  fats  are  split 
into  glycerin  and  fatty  acids.  This  reduction  is  greatly 
aided  by  their  emulsification  in  consequence  of  the  alkalinity 
of  this  juice.  Emulsification  implies  that  the  larger  fat 
globules  are  broken  up  into  a  number  of  much  smaller 
particles  which  are  then  subjected  to  the  cleavage  just 
mentioned.  The  process  of  emulsification,  therefore,  is  really 
a  preliminary  stage  in  the  digestion  of  the  fats.  Saponi- 
fication  then  follows,  because  the  fatty  acids  uniting  with 
the  alkali  of  the  intestinal  contents  form  soaps.  In  this 
soluble  state,  the  fat  is  able  to  traverse  the  lining  cells  of  the 
intestine. 

The  Secretion  of  Pancreatic  Juice. — The  pancreatic 
juice  appears  to  be  produced  at  intervals  upon  the  ejection 
of  chyme  into  the  duodenum.  The  stimuli  responsible  for 
its  secretion  may  be  classified  in  accordance  with  the  same 
general  scheme  as  that  given  above  in  connection  with  the 
formation  of  the  gastric  juice.  It  is  entirely  probable, 
however,  that  the  psychic  element  is  of  slight  importance 
in  this  particular  instance,  and  that  the  formation  of  this 
secretion  is  more  closely  dependent  upon  inherent  stimuli. 
These  local  influences  may  be  either  nervous  or  chemical  in 
nature.  It  is  easily  conceivable  that  the  ejection  of  the 
acid  chyme  into  the  duodenum  must  evoke  certain  reflexes 
which  activate  the  pancreatic  cells.  Thus,  it  has  been 
noted  that  the  flow  of  this  juice  is  accurately  timed,  so  that 
it  takes  place  about  two  hours  after  the  ingestion  of  the 
food  until  the  latter  is  ready  to  leave  the  stomach. 

The  nature  of  this  nervous  reaction  is  not  thoroughly 
understood.  Contrariwise,  a  number  of  important  data 


248  NUTRITION 

have  been  gathered  regarding  the  action  of  the  chemical 
agent.  Thus,  it  has  been  found  that  the  introduction  of  a 
weak  acid  into  the  duodenum  gives  rise  to  a  copious  flow  of 
pancreatic  juice,  and,  peculiarly  enough,  this  result  is  also 
obtained  after  the  nerves  innervating  the  pancreas  have  been 
cut.  Secondly,  it  has  been  noted  that  the  injection  into  the 
bloodstream  of  an  extract  of  the  lining  membrane  of  the 
duodenum  evokes  a  copious  secretion  of  this  juice.  Accord- 
ingly, it  has  been  concluded  that  the  entrance  of  the  acid 
chyme  into  the  duodenum  liberates  a  secretagogue  which 
reaches  the  pancreas  through  the  circulation  and  acts  as  a 
direct  stimulant  to  its  cells.  This  secretagogue  is  retained 
in  the  duodenal  mucosa  in  a  dormant  form  and  requires 
the  chyme  for  its  normal  activation.  The  name  of  secretin 
has  been  applied  to  it.  Like  gastrin,  this  chemical  stimulant 
of  secretion  belongs  to  the  group  of  the  hormones,  peculiar 
products  of  cellular  activity  which  exert  a  chemical  influence 
upon  different  functions  of  our  body. 

The  Liver. — The  liver  is  the  largest  glandular  organ 
in  the  body.  It  weighs  about  1400  to  1700  grams  (50  to  60 
ounces),  and  measures  25  to  30  cm.  (10  to  12  inches)  from 
side  to  side,  15  to  17  cm.  (6  to  7  inches)  in  breadth,  and 
about  7  cm.  (3  inches)  in  thickness.  It  occupies  the  right 
hypochondriac  and  epigastric  regions.  Its  upper  convex 
surface  lies  in  immediate  relation  with  the  diaphragm,  while 
its  lower  surface  touches  the  pyloric  end  of  the  stomach, 
duodenum,  hepatic  flexure  of  the  colon,  and  right  kidney. 
The  entire  organ  is  divided  by  five  grooves  into  a  corre- 
sponding number  of  lobes,  of  which  the  right  is  much  larger 
than  the  left.  Externally,  this  organ  is  enveloped  by  a  layer 
of  connective  tissue,  forming  a  distinct  capsule.  Connective 
tissue  sheaths  and  septa  enter  its  interior,  subdividing  its 
mass  into  numerous  smaller  segments  or  lobules. 

The  individual  lobules  of  this  organ  measure  about  2  mm: 
in  diameter  and  are  composed  of  a  large  number  of  many- 
sided,  nucleated  and  granular  cells  which  are  invested  by  an 
intricate  network  of  capillaries.  The  arrangement  of  the 
latter  will  be  more  easily  understood,  if  it  is  remembered 
that  the  liver  receives  its  bloodsupply  from  two  sources: 


INTESTINAL    DIGESTION  249 

namely,  from  the  hepatic  artery  and  the  portal  vein.  The 
former  is  a  branch  of  the  cceliac  axis  and  conveys  the  blood 
of  the  abdominal  aorta  directly  to  this  organ,  nourishing  its 
framework  but  not  contributing  materially  to  its  store  in 
secretory  material.  As  has  been  mentioned  in  one  of  the 
preceding  chapters,  the  portal  vein  serves  as  the  common 
collecting  channel  for  the  group  of  the  portal  organs,  which 


FIG.  99. — The  liver,  seen  from  below.     1,  inferior  vena  cava;  2,  gall 
bladder.     (Morrow.} 

embraces  the  stomach,  intestine,  pancreas,  and  spleen. 
This  large  venous  tube  enters  the  hilum  of  the  liver,  where 
it  divides  into  a  number  of  smaller  channels,  and  eventually 
into  an  intricate  system  of  capillaries.  These  minute 
tubules  also  receive  the  venous  drainage  from  the  hepatic 
artery,  but  this  type  of  blood  does  not  serve  as  an  important 
source  of  secretory  material  under  normal  circumstances. 

As  is  indicated  in  Fig.  100,  the  portal  terminals  form  a 
ring-like  plexus  of  small  vessels  around  the  periphery  of 
each  lobule,  whence  the  true  capillaries  strive  radially  toward 
its  center.  The  hepatic  cells  lie  closely  packed  in  the  spaces 
between  these  capillaries.  The  venous  collecting  channel 


250 


NUTRITION 


of  each  lobule,  or  intralobular  vein,  unites  with  others  to 
form  the  hepatic  vein  which  eventually  pours  its  contents 
into  the  inferior  vena  cava.  It  need  scarcely  be  emphasized 
that  the  function  of  the  liver  resides  in  the  hepatic  cells 
which  derive  their  secretory  material  chiefly  from  the  portal 
bloodstream. 


FIG.  100. — Diagrammatic  representation  of  the  blood  supply  of  the 
liver  acini.  P,  portal  terminal;  JV,  interlobular  veins;  CV,  central 
veins  which  are  eventually  collected  in  the  hepatic  vein;  HA,  hepatic 
arteriole,  the  interlohular  capillaries  of  which  empty  into  the  portal 
terminals;  B,  biliary  capillary  which  begins  as  biliary  space  between  the 
hepatic  cells. 

The  Functions  of  the  Liver. — While  we  are  now  mainly 
concerned  with  the  digestive  secretion  of  the  liver,  it  seems 
advantageous  to  amplify  this  discussion  by  briefly  summariz- 
ing all  the  important  functions  of  this  organ.  Inasmuch  as 
the  liver  lies  directly  in  the  path  of  the  portal  blood  which  is 
heavily  laden  with  the  products  of  intestinal  digestion,  it 


INTESTINAL    DIGESTION  251 

need  not  surprise  us  to  find  that  it  exerts  a  most  powerful 
influence  upon  all  the  metabolic  functions  of  the  body.  Of 
principal  importance  to  us  at  this  time  are  the  data  which 
prove  that  this  organ: 

(a)  Furnishes  an  internal  secretory  product  which  plays 
an  important  part  in  the  metabolism  of  the  carbohydrates, 
because  it  converts  the  sugar  of  the  portal  blood  into  gly- 
cogen.     In  this  form  the  sugar  is  stored  in  the  hepatic  cells 
until  needed  by  the  cells  of  the  different  tissues.     At  this 
time  the  glycogen  is  reconverted  into  simple   circulating 
sugar; 

(b)  Forms  those  bodies  of  protein   cleavage  which   are 
later  on  excreted  by  the  kidneys  in  the  form  of  urea; 

(c)  Contains  certain  cellular  elements  by  which  the  red 
blood  corpuscles  are  destroyed; 

(d)  Plays  an  important  part  in  the  coagulation  of  the 
blood,  because  it  gives  rise  to  certain  bodies  which  retard 
this  process; 

(e)  Forms  an  important  external  secretion,  the  bile,  which 
aids  in  the  cleavage  of  the  fats;  and 

(/)  Conserves  the  body-temperature,  because  it  produces 
a  considerable  amount  of  heat  and  protects  the  large  ab- 
dominal vessels  against  an  undue  loss  of  heat. 

The  bile  is  a  slightly  alkaline  fluid,  of  decidedly  bitter 
taste  and  high  degree  of  viscidity.  It  possesses  a  golden 
yellow  color  in  the  carnivora  and  a  greenish  color  in  the 
herbivora.  Its  appearance,  however,  changes  very  quickly 
on  exposure,  because  its  pigments  are  easily  oxidized.  The 
biliary  coloring  material  of  the  carnivora  is  called  bilirubin 
and  that  of  the  herbivora,  biliverdin.  In  addition  to  these 
constituents  the  secretion  also  embraces  certain  compounds 
of  sodium  with  organic  acids  which  are  called  bile-salts,  as 
well  as  a  number  of  inorganic  salts  and  cholesterin.  It  is  to 
be  noted  especially  that  the  bile  contained  in  the  smaller 
biliary  passages  is  very  watery,  while  that  in  the  outer 
channels  and  gall-bladder  is  very  viscid.  The  substance 
chiefly  responsible  for  this  change  in  its  consistency  is  mucin, 
a  product  of  the  cells  lining  the  larger  ducts  and  gall-bladder. 
Owing  to  its  peculiar  physical  properties,  this  admixture 


252  NUTRITION 

serves  to  retard  the  progress  of  the  food  through  the  intestine, 
so  that  a  more  thorough  separation  of  the  nutritive  sub- 
stances may  be  effected.  Secondly,  the  mucin  serves  as  a 
protective  and  lubricating  agent  for  the  lining  cells  of  the 
intestine. 

The  digestive  function  of  bile  is  relatively  unimportant, 
because  it  does  not  contain  an  enzyme;  however,  since  it 
is  alkaline  in  its  reaction,  it  must  aid  in  the  emulsification 
and  saponification  of  the  fats.  In  this  regard,  therefore, 
it  may  be  considered  as  an  adjunct  to  the  pancreatic  juice. 
It  is  stated  further  that  it  possesses  very  decided  antiseptic 
properties,  but  this  action  seems  to  be  brought  about  in  an 
indirect  way,  because  it  exerts  a  marked  stimulating  influ- 
ence upon  intestinal  peristalsis.  Inasmuch  as  an  increased 
peristalsis  hastens  the  onward  movement  and  discharge  of 
the  faeces,  the  bacteria  inhabiting  the  intestinal  canal  cannot 
multiply  so  profusely  as  when  the  progress  of  the  faecal  mate- 
rial is  slow.  This  fact  is  clearly  proven  by  the  changed 
character  of  the  intestinal  contents,  following  those  lesions 
of  the  liver  and  its  excretory  ducts  which  cause  a  stoppage 
in  the  inflow  of  bile.  The  faeces  then  become  very  sticky 
and  odoriferous,  while  their  color  gradually  changes  to  gray. 
These  alterations  are  easily  explained,  because  in  the  ab- 
sence of  bile,  a  large  portion  of  the  fats  remains  undigested, 
while,  owing  to  the  diminished  peristalsis,  the  processes  of 
putrefaction  may  continue  for  a  much  longer  time  than  is 
usual.  The  change  in  color  is  due,  of  course,  to  the  loss  of 
the  biliary  pigments. 

The  Storage  of  Bile. — The  bile  is  secreted  constantly, 
although  it  is  not  allowed  to  pass  into  the  duodenum  in  a 
steady  stream.  It  will  be  noted  that  the  hepatic  dud  unites 
at  some  distance  from  the  hilum  of  the  liver  with  the  cystic 
duct  to  form  the  common  bile  duct.  As  has  been  stated  above, 
the  latter  opens  into  the  duodenum  in  conjunction  with  the 
duct  of  Wirsung  of  the  pancreas.  It  is  to  be  observed  that 
the  orifice  of  the  common  bile  duct  is  guarded  by  a  sphincter 
which  is  kept  closed  until  the  chyme  has  been  ejected 
into  the  duodenum.  Consequently,  since  the  bile  emerging 
from  the  hepatic  duct  cannot  enter  the  intestine,  it  must 


INTESTINAL    DIGESTION  253 

back  up  through  the  cystic  duct  into  the  gall-bladder.  Thus, 
it  may  be  said  that  this  musculo-membranous  pouch  serves 
as  a  reservoir  in  which  the  bile  is  stored  until  needed  in  the 
duodenum.  (Fig.  94)  After  the  ejection  of  the  gastric 
contents,  however,  the  sphincter  of  the  common  duct  relaxes, 
and  allows  the  contracting  gall-bladder  to  evacuate  a  portion 
of  its  bile.  The  reflexes  responsible  for  this  motor  action, 
are  elicited  in  a  nervous  way. 


FIG.  101. — Diagram  to  illustrate  the  relation  between  the  villi  and  the 
crypts  of  Lieberki'ihn.  V,  villus;  G,  goblet  cells  secreting  mucus;  C, 
crypt  of  Lieberki'ihn;  L,  lacteal. 

It  might  also  be  mentioned  that  certain  animals,  such  as 
the  horse,  are  not  in  possession  of  a  gall-bladder  and  hence, 
show  a  relatively  constant  influx  of  bile  into  the  duodenum. 
This  appears  to  be  an  absolute  necessity  in  these  animals, 
because  their  digestive  activities  are  never  at  a  complete 
standstill.  The  presence  of  the  gall-bladder,  however,  is  not 
a  functional  necessity  even  in  man,  because  those  persons 
from  whom  this  organ  has  been  removed  to  remedy  a  certain 
abnormal  condition,  do  not  present  any  permanent  disturb- 
ances. It  is  entirely  probable  that  the  larger  bile  ducts  then 


254  NUTRITION 

assume  the  function  of  a  reservoir.  Brief  reference  should 
also  be  made  at  this  time  to  those  conditions  which  lead  to  a 
stagnation  of  the  bile  in  the  gall-bladder  and  biliary  passages. 
These  effects  are  most  frequently  caused  by  the  impaction 
of  gall-stones  or  mucous  plugs  in  the  large  biliary  channels. 
The  steadily  increasing  stagnation  of  the  bile  eventually 
allows  the  coloring  material  of  this  secretion  to  pass  into  the 
lymphatics,  whence  it  reaches  the  general  circulatory  system. 
It  is  then  deposited  in  the  different  tissues,,  imparting  to 
them  a  yellowish  color.  This  condition  is  known  as  jaundice 
or  icterus. 

The  Intestinal  Juice. — The  mucous  lining  of  the  small 
intestine  contains  a  large  number  of  tubular  glands  which 
are  situated  in  the  depressions  between  the  neighboring  villi. 
They  are  known  as  the  glands  of  Lieberkuhn.  While  these 
structures  are  also  present  in  the  large  intestine,  they  assume 
here  a  more  superficial  position,  because  the  mucosa  of  this 
segment  of  the  alimentary  canal  is  free  from  villi.  Further- 
more, their  secretion  no  longer  possesses  a  true  digestive 
action,  but  merely  serves,  by  virtue  of  its  large  content  in 
mucin,  as  a  protective  and  lubricating  fluid. 

Each  gland  of  Lieberkuhn  is  lined  with  a  layer  of  cubical 
cells,  somewhat  different  in  their  general  appearance  from 
those  limiting  the  villi.  The  secretion  furnished  by  these 
cells  contains  several  ferments.  One  of  these,  known  as 
erepsin,  is  proteolytic  in  nature,  while  its  invertase  and  maltase 
act  upon  the  complex  sugars  changing  them  into  glucose. 


CHAPTER  XXV 

THE   PROGRESS    OF   THE   FOOD   THROUGH   THE 
INTESTINES— ABSORPTION 

Movements  of  the  Small  Intestine. — The  movements 
taking  place  in  the  small  intestine  are  of  two  kinds :  namely, 
pendular  and  peristaltic.  Very  soon  after  the  chyme  has  been 
ejected  into  the  duodenum,  it  is  subjected  to  a  minute 
subdivision  by  the  smooth  musculature  of  this  particular- 
portion  of  the  alimentary  canal.  This  action  consists  in  an 
alternate  constriction  and  relaxation  of  its  consecutive 


C^- -2> 

^CPOCDCDCDp 

*  ^-T-V    /-T"N  /^~r*\  ^— r-v    ^£L     * 


AO 


FIG.  102. — Diagram  to  show  the  effect  of  the  rhythmical  constricting 
movements  of  the  small  intestine  upon  the  contained  food  A  string  of 
food  (1)  is  divided  suddenly  into  a  series  of  segments  (2);  each  of  the 
latter  is  again  divided  and  the  process  is  repeated  a  number  of  times 
(3  and  4) .  Eventually  a  peristaltic  wave  sweeps  these  segments  forward 
a  certain  distance  and  gathers  them  again  into  a  long  string,  as  in  (1). 
The  process  of  segmentation  is  then  repeated  as  described  above. 
(Cannon.) 

segments.  Thus,  while  one  of  its  band-like  portions  passes 
into  the  state  of  contraction,  the  one  nearest  to  it  remains 
perfectly  flaccid.  A  moment  thereafter,  however,  the 
previously  constricted  segment  relaxes,  while  the  flaccid 
one  contracts.  The  adjective  " pendular"  has  been  applied 
to  this  movement,  because  it  swings  back  and  forth  from 
loop  to  loop  like  the  pendulum  of  a  clock.  It  has  also  been 

255 


256 


NUTRITION 


characterized  as  rhythmic,  because  it  recurs  at  rather  regular 
intervals. 

As  a  result  of  this  alternate  constriction  and  relaxation,  the 
larger  masses  of  food  are  constantly  broken  up  into  smaller 
pieces  and  thoroughly  moistened  with  the  intestinal  juices 
(Fig.  102).  Later  on,  when  the  intensity  of  this  movement 
diminishes,  the  smaller  portions  are  again  united  into  a  single 
one.  This  rebuilt  mass  of  nutritive  material  is  then  moved 
onward  by  waves  of  peristalsis,  pursuing  a  course  from  above 
^  7^>>,  downward.  At  a  somewhat  lower 

f*  J  level  of  the  small  intestine  it  is 

Jpt-  Coffrtr*  again  subjected  to  this  rhythmic 

f  ^^i     S|  segmentation,  and  so  on  until  the 

P^jflg  distal  portion  of  the  ileum  has 

I  H        MK£|^^  been   reached.      The   peristaltic 

— ""TSr  I^^L  waves  occurring  in  the  small  in- 
^  testine,  always  pass  from  above 
downward,  and  may  involve 
either  a  short  or  a  long  segment 
of  this  gut.  This  form  of 
movement  is  known  as  regular 
peristalsis.  Waves  moving  in 
the  opposite  direction  are  not 
observed  in  the  small  intestine 
under  normal  circumstances, 
although  they  may  arise  in  con- 
sequence of  obstructions  to  the  faecal  material  by  tumors 
and  adhesions. 

Movements  of  the  Large  Intestine. — The  movements  dis- 
cernible in  the  large  intestine  present  the  same  general  char- 
acteristics as  those  occurring  in  the  small  intestine.  It  is  to 
be  emphasized,  however,  that  the  anti-peristaltic  motions, 
consisting  of  waves  progressing  in  a  direction  from  below 
upward,  are  here  more  conspicuous  than  the  regular  ones. 
Even  a  casual  observation  will  show  that  this  portion  of  the 
alimentary  canal  is  not  equally  active  throughout,  for  while 
the  caecum  and  ascending  colon  show  an  almost  constant  peri- 
stalsis, the  transverse  and  descending  colons  are  relatively 
quiescent.  When  the  contents  of  the  ileum  escape  through 


FIG.  103. — Csecum  with  part 
of  the  ventral   wall  removed. 

A,  ileocaecal  valve  and  orifice; 

B,  appendicular   orifice.     (Ra- 
dasch.) 


PROGRESS    OF    FOOD    THROUGH   THE    INTESTINES       257 

the  ileo-caecal  orifice  into  the  caecum,  they  are  still  in  a  fluid 
condition.  Their  content  in  water,  however,  is  greatly 
diminished  during  their  passage  through  this  gut,  so  that 
they  reach  the  transverse  colon  in  a  much  more  solid  state. 

This  absorption  of  water  is  greatly  facilitated  by  the  fact 
that  the  faecal  material  is  not  allowed  to  pass  right  on,  but  is 
moved  forward  and  backward  repeatedly  without  being 
able  to  gain  the  transverse  colon.  After  its  entrance  into 
the  caecum,  it  is  grasped  by  regular  peristaltic  waves  and 
moved  upward  into  the  hepatic  flexure.  Here  it  meets 
with  waves  pursuing  an  opposite  course,  and  is  again  carried 
into  the  pit  of  the  caecum.  This  movement  is  repeated 
many  times  until  relatively  water-free  portions  of  this 
material  finally  succeed  in  passing  over  into  the  transverse 
colon. 

The  transverse  portion  of  the  colon  serves  chiefly  as  a 
storehouse  for  the  faeces.  Here  they  accumulate  throughout 
the  day,  and  are  eventually  removed  from  it  by  regular 
peristaltic  waves  which  force  them  through  the  descending 
colon  into  the  rectum.  As  a  rule,  these  movements  are  few 
in  number  and  occur  during  the  early  hours  of  the  morning, 
when  the  entire  system  after  its  long  period  of  rest,  is  most 
sensitive  to  mechanical  impacts  and  the  effects  of  gravity. 
As  this  material  accumulates  in  the  rectum,  it  exerts  a 
mechanical  influence  upon  the  mucous  lining  of  this  part  in 
consequence  of  which  certain  reflexes  are  evoked  giving  rise 
to  the  act  of  defecation.  This  process  consists  of  two  simul- 
taneous muscular  reactions:  namely,  the  relaxation  of  the 
internal  and  external  sphincters  guarding  the  orifice  of  the 
rectum,  and  the  contraction  of  the  smooth  muscle  cells 
situated  in  its  wall.  The  expulsion  of  the  faeces  may  be 
considerably  hastened  by  the  contraction  of  the  muscles  of 
the  abdominal  wall,  provided  the  glottis  is  kept  closed.  If 
these  reflexes  are  antagonized  by  volition,  they  may  lose 
their  normal  intensity  and  presently  fail  to  develop  alto- 
gether. The  rectum  then  becomes  highly  distended  with 
faecal  material  without  giving  the  sensations  of  fullness  and 
discomfort.  Further,  the  repeated  volitional  interference 
with  these  reflexes  generally  results  in  a  disturbance  in  the 

17 


258 


NUTRITION 


function  of  this  mechanism  which  can  only  be  remedied 
with  difficulty. 

The  Progress  of  the  Food  Through  The  Intestines. — It 
has  been  stated  above  that  the  contents  of  the  stomach  are 
evacuated  at  intervals,  and  that  a  normal  organ  should 


FIG.  104. — Shadows  of  the  human  large  intestine  obtained  by  means  of 
the  Rontgen  rays.  I,  Entrance  of  the  contents  of  the  ileum  into  the 
cecum  and  colon.  II,  the  material  has  progressed  through  the  transverse 
colon  as  far  as  the  splenic  flexure,  some  has  escaped  into  rectum.  Ill,  the 
large  intestine  outlined  by  means  of  a  solution  of  sub-nitrate  of  bismuth 
injected  through  the  rectum. 

empty  itself  in  less  than  five  hours  after  the  ingestion  of  a 
mixed  meal  of  ordinary  bulk.  The  small  intestine  is  trav- 
ersed in  about  five  hours,  although  this  portion  of  the  ali- 
mentary canal  is  about  25  feet  in  length.  Markedly  different 
conditions  are  met  with  in  the  large  intestine,  because  while 
this  segment  is  only  about  6  feet  in  length,  the  food  requires 
almost  20  hours  in  its  journey  to  the  rectum.  It  has  been 


PROGRESS    OF    FOOD    THROUGH    THE    INTESTINES       259 

mentioned  above  that  this  entire  mechanism  is  adjusted  in 
such  a  way  that  the  food  consumed  in  the  course  of  the  pre- 
ceding day,  is  evacuated  during  the  early  hours  of  the 
following  morning.  It  is  true,  however,  that  exceptions 
to  this  rule  are  not  uncommon,  and  are  due  to  differences  in 
the  character  of  the  food  and  the  functional  power  of  the 
intestine  as  well  as  of  that  of  the  entire  body. 

The  Formation  of  the  Faeces. — The  function  of  the  small 
intestine  is  to  separate  the  useful  from  the  useless  constitu- 
ents of  the  food.  The  former  are  immediately  transferred 
into  the  absorbing  channels  of  the  body,  while  the  latter  are 
allowed  to  escape  through  the  ileo-csecal  orifice  into  the 
large  intestine  to  be  eventually  included  in  the  faeces. 
Almost  every  food  contains  a  certain  amount  of  indigestible 
material  which  is  made  up  chiefly  of  the  cellulose  of  the  walls 
of  the  plants  and  the  dense  fibrous  constituents  of  connective 
tissue.  In  addition,  the  large  intestine  receives  that  portion 
of  the  proteins,  carbohydrates  and  fats  which  has  escaped 
digestion,  as  well  as  a  certain  amount  of  excretory  material 
derived  principally  from  the  bile.  The  faeces  also  contain 
enormous  numbers  of  bacteria,  living  as  well  as  dead,  and  in 
addition  a  few  lining  cells  that  have  been  torn  loose  from  the 
mucosa. 

Under  normal  conditions,  however,  only  about  5  to  10  per 
cent,  of  the  food  ingested  is  allowed  to  escape  into  the  large 
intestine,  where  it  is  subjected  to  fermentative  changes 
yielding  a  small  proportion  of  absorbable  material.  This 
is  taken  up  before  it  actually  reaches  the  transverse  colon. 
Even  the  cellulose  may  be  decomposed  to  some  extent  by 
bacterial  action,  allowing  the  proteins  and  carbohydrates 
to  become  separated  from  their  capsular  investments  and  to 
be  chemically  reduced  by  fermentation.  In  the  herbivora, 
somewhat  different  conditions,  are  met  with,  because  their 
food  consists  chiefly  of  cellulose  material,  in  the  meshes  of 
which  different  nutritive  substances  are  contained.  Thus, 
it  will  be  noted  that  the  beginning  portion  of  the  large 
intestine  of  these  animals  is  often  excessively  distended  with 
grasses  and  leaves  undergoing  prolonged  fermentative 
changes  before  they  are  actually  passed  on. 


260  NUTRITION 

The  conditions  in  the  lower  bowel  are  well  adapted  to 
bacterial  life  and  growth.  The  products  derived  from  their 
action  upon  the  remnants  of  the  food  are  in  most  instances 
perfectly  harmless.  Nevertheless,  it  cannot  be  denied  that 
several  of  them  are  capable  of  evoking  certain  disturbing 
general  symptoms,  such  as  dizziness,  muscular  tremors, 
headache,  and  fatigue.  The  most  poisonous  of  these  com- 
pounds find  their  origin  in  the  decomposition  and  putrefac- 
tion of  the  proteins.  In  remedying  this  difficulty,  it  should 
be  our  endeavor  to  increase  the  peristaltic  activity  of  the 
intestines,  so  that  the  bacteria  cannot  multiply  so  rapidly. 
Naturally,  the  intake  of  protein  foods  should  be  reduced  to  a 
minimum.  It  must  be  evident  that  the  administration  of  a 
cathartic  can  afford  only  temporary  relief,  because  the  quick 
evacuation  of  the  f  seces  does  not  necessarily  produce  a  perma- 
nent augmentation  of  the  tonus  of  the  intestinal  musculature. 
This  end  can  only  be  attained  by  general  hygienic  measures. 

Absorption. — The  term  absorption  refers  to  the  passage 
of  the  simplified  foodstuffs  through  the  lining  cells  of  the 
alimentary  canal.  These  cells  form  an  animal  membrane, 
separating  the  contents  of  the  digestive  tract  from  the  general 
fluids  of  the  body.  Accordingly,  this  subject-matter  is 
chiefly  concerned  with  the  processes  which  enable  the  prod- 
ucts of  digestion  to  reach  the  two  absorbing  channels  of 
the  body :  namely,  the  capillaries  of  the  portal  system  and  the 
lacteals.  The  principal  organ  of  absorption  is  the  small 
intestine,  but  certain  substances,  such  as  sugar  and  peptones, 
may  also  traverse  the  lining  of  the  stomach  as  well  as  that 
of  the  colon.  In  general,  however,  it  may  be  stated  that  the 
absorption  of  the  simplified  foodstuffs  has  been  completed 
by  the  time  the  ileo-csecal  orifice  is  reached.  Even  water 
follows  this  path,  and  only  a  relatively  small  proportion  of 
it  escapes  into  the  large  intestine.  Practically  none  is 
absorbed  in  the  stomach.  The  inorganic  salts  behave  in  the 
same  way  as  water. 

The  foregoing  discussion  pertaining  to  the  cleavage  of 
the  different  foodstuffs,  must  have  shown  that  the  lumen  of 
the  small  intestine  contains:  (a)  simple  sugars  derived  from 
the  various  carbohydrates;  (6)  amino-acids,  as  products  of 


PROGRESS   OF   FOOD   THROUGH   THE    INTESTINES       261 

protein  digestion;  and  (c)  emulsified  and  saponified  fats. 
In  these  forms  the  various  foodstuffs  are  able  to  traverse  the 
intestinal  wall  and  to  enter  the  absorbing  channels.  Curi- 
ously enough,  by  far  the  largest  amount  of  the  fat  absorbed 
selects  the  lacteals  as  its  specific  channel  of  entry.  It  has 
been  mentioned  above  that  the  lymph  then  assumes  the 
character  of  an  oil  emulsion  and  becomes  milky  in  appear- 
ance. Lymph  loaded  with  these  minute  globules  of  fat  is 
called  chyle.  At  this  time,  even  the  blood  presents  an 
oily  appearance,  which  is  due  to  the  steady  inflow  of  chyle 
from  the  thoracic  duct.  It  will  be  remembered  that  this 
channel  collects  the  lymph  from  all  the  lacteals,  and  conveys 
it  directly  into  the  blood  of  the  left  subclavian  vein.  The 
sugars  and  amino-acids  are  conveyed  into  the  blood-capil- 
laries of  the  intestine,  whence  they  reach  the  liver  by  way 
of  the  portal  vein  and  its  terminal  branches. 

The  Intestinal  Lining  Cells  as  Factors  in  Absorption. — 
The  mucous  membrane  of  the  intestine  is  composed  of  in- 
numerable cells  arranged  side  by  side.  Their  outer  surfaces 
lie  in  relation  with  the  digested  foodstuffs,  whereas  their 
inner  surfaces  border  upon  the  body-fluids,  the  blood  and 
lymph.  These  cells,  therefore,  form  a  living  membrane, 
through  which  the  products  of  digestion  must  pass  in  order 
to  reach  the  circulatory  channels  of  the  body.  In  its  normal 
state  this  membrane  is  semi-permeable,  i.e.,  it  allows  certain 
substances  to  pass  and  not  others.  It  will  be  evident  that 
the  other  two  types  of  membranes:  namely,  the  permeable 
and  impermeable  ones,  need  not  be  considered  in  this  con- 
nection, because  the  former  permit  the  passage  not  only  of 
water,  but  also  of  the  substances  dissolved  therein,  while  the 
latter  are  completely  impervious  to  both. 

The  question  may  then  be  asked,  whether  these  lining 
cells  exert  a  vital  selective  action  upon  the  substances  con- 
tained in  the  intestinal  canal,  or  simply  permit  them  to 
traverse  their  cytoplasm  in  accordance  with  ordinary  physical 
laws.  It  seems  proper  for  us  to  take  a  more  conservative 
view  than  either,  and  to  state  that  while  the  laws  of  pressure, 
diffusion  and  osmosis  play  an  important  part  in  absorption, 
several  of  the  phenomena  connected  with  this  process  cannot 


262  NUTRITION 

be  fully  explained  upon  this  simple  mechanistic  basis.  Con- 
sequently, it  may  be  assumed  that  these  lining  cells  play 
the  part  of  minute  laboratories  and  are  able  to  force  the 
nutritive  substances  through  their  cytoplasm  in  an  active 
way,  modifying  them  in  their  passage  in  such  a  way  that 
they  become  available  for  tissue  metabolism.  The  nature 
of  the  chemico-physical  processes  occurring  within  these 
cells,  is  not  clearly  understood  as  yet  and  hence,  it  seems 
permissible  to  designate  them  by  the  general  term  of  vital 
activity  or  vitalismus.  Accordingly,  it  may  be  concluded 
that  the  absorption  of  the  foodstuffs  depends  upon  differences 
in  pressure,  diffusion  and  osmosis,  as  well  as  upon  certain 
inherent  physical  and  chemical  powers  of  these  lining  cells. 

In  harmony  with  the  preceding  statement,  the  lining  cells 
of  the  intestines  may  be  compared  to  glands  with  this  differ- 
ence, however,  that  they  secrete  from  without  inward. 
They  select  various  substances  from  the  contents  of  the 
intestinal  canal  and  transfer  them  eventually  into  the  ab- 
sorbing channels.  In  several  instances,  this  transfer  is 
associated  with  true  constructive  changes.  The  simple 
products  of  digestion  are  rebuilt  into  complex  compounds 
akin  to  those  found  in  the  different  tissues  of  the  body.  This 
is  true  especially  of  the  fats,  the  glycerin  and  acids  of  which 
are  reconstructed  during  their  journey  through  the  intestinal 
mucosa  into  a  form  practically  identical  with  that  of  the 
neutral  fats  of  the  body.  In  other  words,  these  cells  are 
able  to  synthetize  the  products  of  the  cleavage  of  this  food- 
stuff. The  nature  of  this  synthesis  is  as  yet  unknown. 

Diffusion,  Osmosis,  Dialysis. — The  passage  of  water  and 
salts  may  be  explained  in  accordance  with  the  ordinary  physi- 
cal laws  of  diffusion  and  osmosis.  If  a  solution  of  a  salt  is 
placed  in  a  receptacle  and  a  layer  of  water  is  allowed  to  run 
over  it,  it  will  be  found  after  a  time  that  a  certain  number  of 
molecules  of  the  salt  have  passed  into  the  overlying  water, 
thereby  establishing  a  medium  of  uniform  composition. 
Such  an  interchange  also  takes  place  if  two  solutions  of  dif- 
ferent salts  are  brought  in  relation  with  one  another.  This 
spreading  about  or  scattering  of  the  molecules  through 
suitable  media  is  termed  diffusion. 


PROGRESS   OF   FOOD    THROUGH   THE    INTESTINES       263 


If  we  now  interpose  a  semi-permeable  membrane  between 
the  salt  solution  and  the  water,  the  molecules  of  the  water 
will  gradually  pass  through  the  pores  of  this  septum  into 
the  solution,  raising  the  level  of  the  latter.  This  movement 
of  water  from  one  side  of  the  membrane  to  the  other  in 
response  to  differences  in  the  molecular  concentration  of  the 
fluids  is  known  as  osmosis.  It  may 
be  illustrated  by  filling  a  thistle  tube 
with  a  solution  of  magnesium  sul- 
phate; closing  its  orifice  with  an 
animal  membrane,  and  immersing 
the  entire  cup  in  water  so  that  the 
level  of  the  latter  corresponds  pre- 
cisely with  that  of  the  salt  solution. 
This  contrivance  is  known,  as  an 
osmometer.  After  several  hours  it 
will  be  found  that  the  salt  solution 
in  the  thistle  tube  has  risen  to  a 
considerable  height,  proving  thereby 
that  an  appreciable  back  pressure 
has  been  established  against  the 
membrane.  This  pressure,  which  is 
termed  osmotic  pressure,  is  responsi- 
ble for  the  inflow  of  the  molecules  FlG-  105'TA  simple 

..  ,TT1  .  ,        osmometer.     The  recepta- 

Of    water.      When    magnesium     Sul-      cle  contains  water,  and  the 
phate    Or    epsom    salt   is   Used   as   a      cell    a   solution  of  magne- 

cathartic,   it   causes  a   transfer   of     *&SSfiF'.*£   « 
water    into    the    intestinal    canal,     drawn  through  the  semi- 

thereby    rendering    the    faces    more      Permeable  membrane,  the 
n    •  j  level  of  the  solution  rises. 

liuia. 

The  manner  in  which  the  osmotic  pressure  is  produced 
may  be  illustrated  in  the  following  way:  Supposing  that 
we  fill  an  elastic  membranous  ball  with  a  solution  of  cane 
sugar  and  immerse  it  in  water.  If  the  wall  of  this  ball  is 
equipped  with  many  minute  pores  which  are  sufficiently  large 
to  allow  the  molecules  of  water  to  pass  but  too  small  to 
permit  the  escape  of  those  of  cane  sugar,  the  ball  will  eventu- 
ally become  highly  distended.  Obviously,  this  distention 
indicates  that  the  fluid  within  has  been  placed  under  a 


264  NTTTKITION 

high  pressure.  How  does  it  happen  that  although  these 
pores  allow  the  passage  of  the  molecules  of  water  in  both 
directions,  the  number  of  these  molecules  is  constantly 
increased  on  the  inside  of  the  membrane?  Inasmuch  as  the 
ball  is  filled  with  a  solution  of  cane  sugar,  the  number  of 
the  molecules  of  water  present  therein  must  be  smaller 
than  that  in  an  equally  large  volume  of  clear  water.  Sec- 
ondly, since  the  molecules  of  cane  sugar  constantly  hit  the 
pores  but  cannot  get  through,  they  must  prevent  each  time 
the  passage  outward  of  a  molecule  of  water.  Contrariwise, 
no  hindrance  is  placed  in  the  path  of  the  molecules  of  water 
upon  the  outside  of  the  membrane. 

This  process  may  easily  be  reversed  by  placing  the  ball 
containing  the  aforesaid  solution,  in  a  solution  of  twice 
the  strength  of  that  within  it..  The  molecules  of  water  will 
then  be  forced  into  the  outside  compartment  until  the  solu- 
tion without  has  acquired  the  same  concentration  as  that 
within.  The  ball  now  decreases  in  size.  It  will  also  be  seen 
that  if  the  concentration  of  the  solution  within  the  ball 
equals  that  of  the  surrounding  medium,  no  change  in  the 
quantity  of  the  water  within  can  take  place,  because  the 
number  of  pores  hit  by  the  sugar  molecules  is  now  the  same 
on  the  two  sides. 

A  solution  possessing  the  same  concentration  and  osmotic 
pressure  as  the  blood  plasma,  is  said  to  be  isotonic.  Con- 
trariwise, one  showing  a  greater  osmotic  pressure  is 
characterized  as  hypertonic,  and  one  possessing  a  lesser 
osmotic  pressure  as  hypotonic.  While  many  such  simple 
arrangements  actually  exist  in  our  body,  the  most  frequent 
interchanges  are  those  affecting  the  crystalloids  and  colloids. 
The  process  of  interchanging  these  complex  substances  in 
both  directions  through  an  animal  membrane  is  known  as 
dialysis. 


CHAPTER  XXVI 
METABOLISM 

The  Life  History  of  the  Carbohydrates. — While  animal 
foods  are  poor  in  carbohydrates  and  rich  in  proteins,  vege- 
tables usually  contain  copious  amounts  of  carbohydrates, 
and  especially  of  starch.  This  starch  is  generally  contained 
within  the  plant  cells,  the  walls  of  which  are  composed  of 
cellulose  membranes,  impermeable  to  the  digestive  juices. 
For  this  reason,  plant  foods  such  as  cereals,  vegetables, 
fruits  and  nuts,  must  first  be  separated  as  much  as-  possible 
from  their  indigestible  investments  by  the  process  of  thresh- 
ing in  the  cases  of  rye,  barley,  wheat,  and  oats,  and  by  the 
processes  of  shelling,  husking,  and  peeling  in  the  cases  of 
corn,  nuts,  potatoes,  celery,  radishes,  and  similar  nutritive 
bodies.  These  procedures  having  been  completed,  these 
foods  are  subjected  to  further  refining  by  winnowing,  grind- 
ing, cleaning,  and  cooking.  The  purpose  of  the  separation 
of  the  useless  from  the  useful  materials  is  to  concentrate  the 
food  before  it  is  actually  subjected  to  the  process  of  digestion. 
In  principle,  however,  the  purpose  of  the  " internal"  refining 
of  the  food  is  the  same  as  that  of  the  "external,"  because  it 
continues  the  processes  already  started  in  the  field,  mill  and 
kitchen,  finally  cleaving  the  concentrated  foodstuffs  hito 
their  simplest  components. 

The  large  molecules  of  starch  are  first  acted  upon  by  the 
ptyalin  of  the  saliva.  Maltose  is  the  result  of  this  cleavage. 
Together  with  the  simple  sugars,  the  maltose  then  traverses 
the  stomach  unchanged  by  the  gastric  juice,  and  is  again 
acted  upon  by  the  amylopsin  of  the  pancreatic  juice  as  well 
as  by  the  invertase  of  the  intestinal  secretion.  Dextrose, 
glucose  and  other  sugars  of  the  simplest  order  are  the  result 
of  this  series  of  digestive  events.  Chief  among  these  is 
dextrose  which  appears  to  be  practically  identical  with  the 
sugar  found  in  grapes. 

265 


266  NUTRITION 

It  will  be  remembered  that  the  green  parts  of  the  plants, 
and  especially  the  leaves,  manufacture  sugar  from  the 
carbon  dioxid  of  the  air  and  the  water  of  the  soil,  the  energy 
required  for  this  reaction  being  furnished  by  the  sunlight. 
This  sugar  solution  is  transported  from  cell  to  cell  in  the  form 
of  the  sap  and  serves  as  a  source  of  energy  to  the  plant.  The 
excess  of  sugar  is  converted  into  starch  and  is  stored  in  the 
form  of  granules  in  the  cytoplasm  of  the  different  cells. 
This  stored  energy  serves  as  reserve  fuel  and  is  drawn  upon 
whenever  the  plant  is  not  able  to  form  fresh  sugar.  At  this 
time  the  starch  is  reconverted  into  sugar. 

Let  us  see  whether  we  cannot  discover  a  certain  similarity 
between  this  process  and  that  taking  place  after  the  simple 
sugars  have  traversed  the  intestinal  lining  cells  and  have 
entered  the  bloodstream.  Sugar  serves  as  an  important 
fuel  for  our  tissues,  especially  the  muscle  cells.  It  is  a  well 
known  fact  that  every  contraction  of  muscle  uses  up  a  cer- 
tain amount  of  this  substance,  and  hence,  it  may  be  con- 
jectured that  the  body  must  always  be  well  supplied  with  it. 
Accordingly,  it  is  found  that  the  sugar  conveyed  to  the  liver 
by  the  portal  blood  stream,  is  stored  in  the  cells  of  this 
organ  for  future  use.  Whenever  the  body  is  in  need  of  extra 
amounts  of  sugar,  this  starch-like  body  glycogen  is  recon- 
verted into  simple  circulating  sugar  and  oxidized. 

The  carbohydrate  material  which  is  stored  in  the  cells  of 
the  liver,  is  known  as  animal  starch  or  glycogen.  It  is  more 
like  a  starch  than  a  sugar  and  exhibits,  therefore,  a  limited 
solubility  and  high  molecular  weight.  It  is  found  in  the 
aforesaid  cells  in  the  form  of  delicate  chips  and  its  presence 
is  directly  demonstrable  by  chemical  means.  It  appears, 
therefore,  that  a  portion  of  the  simple  sugars  of  the  portal 
blood  is  condensed  or  dehydrated  into  a  stationary  form,  but 
may  at  any  time  thereafter  be  reconverted  into  its  original 
condition.  Accordingly,  the  formation  of  glycogen  is  acconi- 
plished  by  a  process  the  reverse  of  that  leading  to  the  cleavage 
of  the  starch  and  complex  sugars.  It  is  usually  stated  that 
the  liver  cells  accomplish  this  change  by  means  of  an  inter- 
nal product  or  secretion,  the  nature  of  which  is  unknown. 

A  storage  of  sugar  also  takes  place  in  other  tissues,  prin- 


METABOLISM  267 

cipally  the  skeletal  muscles.  Hence,  the  liver  should  be 
regarded  merely  as  a  central  store  house,  from  which  the 
other  reservoirs  are  supplied  whenever  their  content  in 
glycogen  has  been  depleted.  It  is  to  be  noted,  however, 
that  while  the  percentage  of  sugar  in  the  muscles  is  small, 
their  total  sugar  content  must  be  considerable,  because  the 
entire  mass  of  muscle  tissue  is  very  much  larger  than  that  of 
the  liver.  All  the  tissues  together  store  probably  as  much 
as  1  Ib.  of  glycogen.  The  final  product  of  the  oxidation  of 
sugar  is  carbon  dioxid  and  water. 

Impaired  Carbohydrate  Metabolism. — Because  of  the 
constant  oxidation  of  the  circulatory  sugar  by  the  tissues 
and  the  storage  of  the  superfluous  amounts  of  this  foodstuff 
in  the  form  of  glycogen,  the  blood  is  not  able  to  retain  a 
significant  amount  of  it.  Furthermore,  owing  to  the  afore- 
said means  of  balancing  the  ingo  and  outgo  of  sugar,  its 
percentage  in  the  blood  must  remain  practically  constant. 
But  if  the  ingo  of  sugar  greatly  exceeds  its  outgo  and  an 
ample  amount  of  glycogen  has  been  deposited  in  the 
tissues,  the  body  may  change  its  excess  quantity  into  fat. 
This  "carbohydrate-fat"  differs  somewhat  from  the  ordinary 
tissue-fat  which  is  usually  derived  from  the  fat  of  the  food 
and  in  a  small  measure  also  from  the  excess  proteins.  This 
power  of  the  body  enabling  it  to  form  tissue-fat  from  other 
foodstuffs,  explains  the  fact  that  the  consumption  of  exces- 
sive amounts  of  carbohydrates  is  prone  to  lead  to  corpulence 
and  adiposity.  This  condition  is  common  in  heavy  drinkers 
of  beer  and  malt  extracts,  but,  naturally,  the  alcohol  may 
play  an  important  part  in  the  deposition  of  excessive  amounts 
of  fat,  because  it  is  easily  oxidized  into  carbon  dioxid  and 
water  and  protects,  therefore,  the  carbohydrates  and  fats. 
The  latter  are  thereby  spared  and  may  be  stored  directly. 
Alcohol  as  such,  however,  is  not  a  food.  Its  nutritive  value 
is  entirely  due  to  an  indirect  cause. 

It  has  been  mentioned  above  that  the\pancreas  furnishes  j 
not  only  an  external  secretion,  but  also  an  internal  one  which  is  i 
concerned  with  the  metabolism  of  the  carbohydrates.     This 
internal  product  arises  in  the  cells  of  the  islands  of  Langer- 
hans,  and  possesses  a  peculiar  influence  upon  the  constitution 


268  NUTRITION 

of  the  circulating  sugar.  It  changes  this  substance  into  a 
form  which  can  be  easily  oxidized  by  the  tissue  cells.  Not 
having  been  subjected  to  this  modification,  the  sugar  cannot 
be  utilized  completely  by  the  cells,  and  must  then  accumulate 
in  the  blood  and  eventually  escape  into  the  urine. 

This  lack  of  pancreatic  hormone  and  resulting  inability 
on  the  part  of  the  tissue  cells  to  reduce  the  sugar,  finally 
give  rise  to  a  complex  of  symptoms  characterizing  the  condi- 
tion of  diabetes  mellitus.  As  is  evident  from  the  foregoing 
discussion,  this  disease  is  metabolic  in  its  nature  and  is  not 
due  to  a  derangement  of  kidney  function.  It  is  only 
natural  to  suppose  that  the  accumulation  of  sugar  in  the 
blood  must  finally  force  the  kidneys  to  eliminate  this  sub- 
stance in  an  attempt  to  keep  the  concentration  of  the  blood 
at  its  normal  low  level.  It  should  be  noted,  however,  that 
the  urine  always  contains  slight  amounts  of  sugar  and  albu- 
min, and  hence,  a  distinctly  abnormal  condition  can  only 
arise  when  its  content  in  these  substances  exceeds  a  certain 
low  limit.  Furthermore,  sugar  invariably  escapes  into  the 
urine  whenever  excessive  amounts  of  it  have  been  ingested. 
Even  a  few  pieces  of  candy  may  suffice  at  times  to  produce 
this  effect  when  the  system  has  already  been  well  stocked 
with  sugar.  Obviously,  this  condition  which  is  known  as 
physiological  or  alimentary  glycosuria,  is  only  temporary  in 
its  nature,  while  diabetes  is  characterized  by  a  permanent 
discharge  of  excessive  amounts  of  sugar. 

The  Life  History  of  the  Proteins. — The  most  striking 
difference  between  the  plants  and  animals  consists  in  the 
inability  of  the  latter  to  manufacture  protein  material  from 
non-protein  substances.  Contrariwise,  the  plants  are  able 
to  obtain  certain  salts  from  the  soil  and  to  abstract  carbon, 
hydrogen  and  some  oxygen  from  its  preformed  carbohydrates, 
finally  uniting  them  in  certain  proportions  into  the  complex 
molecule  of  a  protein.  The  nitrogen,  sulphur  and  phosphorus 
are  derived  from  the  salts. 

Such  foods  as  the  white  of  eggs,  meat,  curdled  milk,  beans, 
peas,  etc.  contain  their  own  peculiar  types  of  proteins.  All 
of  them  are  reduced  by  the  different  digestive  fluids  into 
amino-acids  which,  however,  show  certain  differences  in 


METABOLISM  269 

accordance  with  their  origin.  About  twenty  amino-acids 
are  known  at  the  present  time.  These  simple  building- 
stones  are  reunited  in  the  system  into  the  proteins  of  the 
body  which  are  later  employed  by  the  tissues  in  the  con- 
struction of  their  cellular  materials.  The  proteins,  so  to 
speak,  form  the  core  of  all  living  animal  matter. 

We  have  noted  that  the  proteins  are  not  acted  upon  in  the 
mouth,  but  are  changed  in  the  stomach  by  the  pepsin  and 
rennin  into  peptones.  A  further  cleavage  takes  place  in  the 
small  intestine  in  consequence  of  the  action  of  trypsin  and 
erepsin,  but  the  different  amino-acids  resulting  during  this 
last  stage  of  digestion,  by  no  means  possess  the  same  nutritive 
value.  Probably  the  most  useful  are  the  ones  derived  from 
meat,  although  those  of  milk,  potatoes,  and  rice  are  also 
very  valuable.  These  facts  should  be  carefully  borne  in 
mind  when  selecting  a  suitable  diet  for  those  persons  whose 
weight  has  been  greatly  reduced  by  illness.  Under  these 
circumstances  the  carbohydrates  are  attacked  first  and  then 
the  fats,  because  these  foodstuffs  really  protect  the  protein 
core  of  the  body.  Thus,  the  former  will  have  been  fully 
utilized  before  the  proteins  are  actually  affected.  Conse- 
quently, in  rebuilding  .the  constituents  of  the  body  we  must 
first  endeavor  to  furnish  an  adequate  supply  of  proteins 
from  which  the  principal  mass  of  the  cells  can  be  recon- 
structed. Beef  broth  is  the  most  potent  nutrient  that  may 
be  offered  to  the  body  at  this  time.  Later  on,  after  the 
protein  content  of  the  cells  has  been  replaced,  the  body  re- 
plenishes its  store  in  fat  and  sugar. 

Under  ordinary  circumstances,  however,  the  best  results 
are  obtained  when  the  body  is  supplied  with  proteins  of 
different  origin.  Thus,  while  it  is  true  that  meat  is  a  very 
concentrated  protein  food  and  brings  quick  results,  it  may 
be  quite  unable  to  furnish  certain  "building  stones"  which 
are  absolutely  essential  to  the  cells.  It  is  entirely  probable 
that  the  latter  may  require  at  times  the  amino-acids  derived 
from  vegetables.  Obviously,  however,  vegetables  must  be 
eaten  in  very  large  amounts  in  order  to  satisfy  the  protein 
needs  of  the  body,  and,  again,  an  exclusively  vegetarian 
diet  would  be  prone  to  lead  to  a  want  of  meat  proteins. 


270  NUTRITION 

It  should  also  be  noted  that  certain  proteins  are  quite 
unable  to  meet  the  protein  requirements  of  the  body.  Such 
a  food  is  the  connective  tissue  of  uncooked  meat.  It  contains 
collagen  which  is  converted  by  heat  in  the  presence  of  water 
into  soluble  gelatine.  To  this  class  of  albuminoids  also 
belongs  the  protein  of  Indian  corn.  The  amino-acids  derived 
from  cellular  metabolism,  are  finally  converted  into  carbon 
dioxid,  water,  and  several  relatively  simple  substances 
containing  nitrogen.  Chief  among  the  latter  is  urea.  The 
most  considerable  portion  of  this  excretory  material  is 
formed  in  the  liver,  although  it  cannot  be  denied  that  it  may 
also  be  manufactured  by  the  tissues  at  large.  Urea  circu- 
lates in  a  soluble  form,  and  is  finally  removed  from  the  blood 
by  the  cells  of  the  kidneys.  Hence,  these  organs  do  not 
actually  manufacture  it,  but  simply  transfer  it  to  the  urine. 

The  Life  History  of  Fat. — Like  the  carbohydrates,  the 
fats  consist  of  carbon,  hydrogen  and  oxygen,  but  oxygen  is 
present  in  relatively  small  amounts.  They  do  not  contain 
nitrogen  and  are  insoluble  in  water.  Probably  the  most 
familiar  compounds  of  this  kind  are  the  fat  of  meat,  butter, 
olive  oil,  and  lard.  The  saliva  is  powerless  to  act  upon  these 
substances,  and  so  is  in  a  large  measure  the  gastric  juice. 
In  the  small  intestine,  however,  they  are  decomposed  into 
fatty  acids  and  glycerin.  Soaps  are  formed  here  in  the  pres- 
ence of  alkalies.  While  traversing  the  lining  cells  of  the  intes- 
tine, these  simple  elements  are  rebuilt  into  neutral  fat  of  a 
variety  peculiar  to  the  animal.  They  are  then  transferred 
into  the  lymphatics  of  the  villi,  i.e.,  into  the  lacteals.  It  has 
been  stated  above  that  the  contents  of  these  absorbing  chan- 
nels assume  a  milky  appearance  when  loaded  with  the  rebuilt 
globules  of  fat. 

In  general,  it  holds  true  that  the  different  foodstuffs  serve 
a  twofold  purpose  after  their  absorption  into  the  body. 
Thus,  they  may  be  burned  up  directly  by  the  cells  to  yield 
energy  or  may  first  be  stored  as  an  intricate  part  of  their 
cytoplasm.  Concerning  the  fats  it  should  be  mentioned  that 
they  are  finally  reduced  into  their  characteristic  end-products 
carbon  dioxid  and  water.  As  has  already  been  alluded  to, 
a  portion  of  them  always  serves  the  immediate  oxidative 


METABOLISM  271 

needs  of  the  body,  while  another  portion  is  either  stored  as 
such  or  is  modified  to  enter  into  the  combination  of  the  sub- 
stance of  the  cells.  The  former  may  be  termed  circulating 
fat  and  the  latter,  tissue  fat.  A  similar  terminology  may  be 
employed  in  the  cases  of  the  proteins  and  carbohydrates  to 
designate  their  fate. 

When  fully  oxidized  fat  yields  carbon  dioxid  and  water. 
It  serves  as  a  very  important  source  of  energy.  This  being 
the  case,  the  body  always  retains  a  considerable  portion  of 
it  in  the  form  of  stored  fat.  This  depot-fat  is  mobilized  and 
transported  to  the  tissues  whenever  the  latter  have  used  up 
their  own  store  of  energy-yielding  material.  Fat  is  stored 
in  the  so-called  adipose  tissues;  for  example,  in  the  subcutane- 
ous connective  tissue,  where  it  appears  as  globules  within 
the  intracellular  material.  This  depository  is  usually  design- 
ated as  the  panniculus  adiposus.  Other  store  houses  are  the 
shafts  of  the  long  bones,  the  region  around  the  kidney,  the 
mesentery,  and  the  liver.  The  average  human  adult  gives 
lodgment  to  about  10  kg.  of  fat. 


CHAPTER  XXVII 

THE  METABOLIC  REQUIREMENTS  OF  THE  BODY 
ANIMAL  HEAT 

Calorimetry. — Under  ordinary  circumstances,  about  nine- 
tenths  of  the  food  ingested  passes  into  the  absorbing  channels, 
and  eventually  leaves  the  body  in  the  form  of  various 
excretory  products.  It  is  true,  however,  that  the  quantity 
of  the  food  taken  in  is  usually  much  larger  than  is  actually 
required  to  cover  the  metabolic  needs  of  the  body.  This 
ingestion  of  food  in  excess  of  that  necessary  to  balance 
the  waste,  is  called  luxus  consumption.  Obviously,  an 
intake  in  decided  excess  of  actual  requirements  must  lead 
to  a  greater  escape  of  still  useful  substances  into  the  large 
intestine. 

The  food  which  has  been  absorbed,  possesses  a  composition 
rendering  it  well  adapted  for  the  oxidations  taking  place  in 
the  cells  of  the  tissues.  In  principle,  these  processes  do  not 
differ  from  those  constantly  occurring  in  atmospheric  air, 
because  sugar  and  fat  may  also  be  oxidized  outside  the 
body  under  liberation  of  carbon  dioxid  and  water.  It  is 
true,  however,  that  the  cells  are  able  to  make  a  greater  eco- 
nomical use  of  these  materials  than  could  be  obtained  by 
their  combustion  outside  the  body. 

We  are  well  aware  of  the  fact  that  oxidations  and  com- 
bustions yield  heat,  and  that  the  value  of  any  combustible 
material,  such  as  wood  or  coal,  may  be  ascertained  directly 
by  measuring  the  heat  derived  from  it  when  burned  in  the 
presence  of  oxygen.  Very  similar  tests  may  be  made  with 
the  different  foodstuffs.  A  measured  quantity  of  them-  is 
placed  in  a  special  metal  receptacle  and  reduced  in  the 
presence  of  oxygen.  The  term  of  bomb-calorimeter  is  applied 
to  these  simple  heat  indicators.  Besides,  it  is  possible  to 
ascertain  the  heat  given  off  by  an  animal  during  a  certain 

272 


METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL   HEAT     273 

period  of  time  by  placing  it  in  an  apparatus  which  is  known 
as  a  calorimeter.  The  animal  may  then  be  fed  with  the 
different  foodstuffs  to  determine  their  value  in  terms  of  heat 
liberated  by  it.  In  its  most  complete  form  an  instrument  of 
this  kind  consists  of  an  air-tight  chamber  which  is  sur- 


CT 


FIG.  106. — Water  calorimeter.  A,  inner  compartment  for  animal; 
SH,  space  filled  with  non-oonductile  material;  ENT  and  EXT,  tubes 
for  the  respiratory  air;  CT,  thermometer  in  jacket  filled  with  water; 
S,  stirrer  to  equalize  the  temperature  of  the  water.  (Reichert.) 


rounded  by  a  jacket  filled  with  water.  The  temperature  of 
the  water  is  measured  by  means  of  a  thermometer.  In 
order  to  protect  this  entire  apparatus  against  undue  loss  of 
heat,  it  is  invested  with  a  layer  of  felt,  a  material  conducting 
heat  very  poorly.  The  inside  compartment  is  connected 
by  means  of  a  tube  with  a  respiration  apparatus,  furnishing 
a  definite  quantity  of  air  in  a  given  period  of  time.  Further- 

18 


274 


NUTRITION 


more,  the  ingoing  and  outgoing  air  may  be  analyzed  in  order 
to  be  able  to  determine  the  amount  of  carbon  dioxid  given 
off  by  the  animal  during  its  stay  in  the  calorimeter.  In 
addition,  the  character  and  composition  of  the  ingesta  and 
excreta  are  carefully  noted  so  as  to  be  able  to  ascertain  the 
energy  supply  and  waste.  In  recent  years  the  size  of  these 
compartments  has  been  increased  sufficiently  to  accommo- 


SCALE      I     METER 


FIG.  107. — Schematic  outline  of  the  respiration  calorimeter.  A, 
dead  air  space  between  copper  and  zinc  walls;  B,  dead  air  space  between 
zinc  wall  and  wooden  wall ;  C,  dead  air  space  between  inner  and  outer 
wooden  walls.  E,  tube  for  food;  Sand  H,  inlet  and  outlet  for  water;  A, 
air  circulation.  (Atwater  and  Benedict.) 

date  persons  whose  metabolic  requirements  are  to  be  tested. 
These  large  respiration-calorimeters  possess  the  general 
•  aaracter  of  a  small  room,  in  which  the  person  selected  for 
the  experiment  may  move  about  and  be  subjected  to  various 
tests  related  to  the  subject  of  inquiry  Thus,  he  may  fte 
made  to  ride  a  stationary  bicycle  in  order  to  determine  the 
metabolic  changes  evoked  by  perfectly  definite  amounts  of 
work. 

The  Calorie. — We  have  previously  noted  that  the  amount 
of  mechanical  energy  liberated  by  a  person  may  be  expressed 


METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL    HEAT     275 

in  terms  of  work.  Thus,  we  speak  of  the  work  performed  by 
lifting  a  weight  of  one  kilogram  to  a  height  of  one  meter  as  a 
kilogrammeter.  In  seeking  an  equally  exact  unit  of  measure- 
ment for  the  energy  liberated  in  the  form  of  heat,  use  is 
commonly  made  of  the  large  calorie,  i.e.,  the  amount  of 
heat  required  to  raise  the  temperature  of  one  kilogram  of 
water  by  one  degree  centigrade.  We  also  speak  at  times 
of  the  small  calorie,  which  refers  to  the  amount  of  heat 
needed  to  raise  the  temperature  of  one  gram  of  water  by 
one  degree  of  centigrade.  Supposing,  therefore,  that  the 
quantity  of  water  in  the  calorimeter  weighs  10  kg.  and  that 
its  temperature  rises  1°  C.  every  half -hour,  the  amount  of 
heat  liberated  by  the  animal  during  this  time,  amounts  to 
10  calories,  or  to  480  calories  in  the  course  of  twenty-four 
hours. 

The  Energy  in  Food. — Inasmuch  as  the  body  is  never  in 
a  condition  of  complete  rest,  it  cannot  justly  be  compared  to 
a  machine.  We  know  that  the  latter  does  not  consume 
energy  when  its  movements  are  arrested.  The  body  simply 
shows  phases  of  comparative  rest  alternating  with  periods 
of  activity,  during  which  the  heart,  respiratory  organs,  and 
glands  continue  to  fulfill  at  least  a  part  of  their  usual  duties. 
Consequently,  the  evolution  of  heat  can  only  be  determined 
with  accuracy  when  the  aforesaid  tests  are  continued  for  long 
periods  of  time. 

It  should  also  be  remembered  that  the  balancing  of  the 
ingo  and  outgo  is  a  relatively  simple  matter  in  the  cases  of 
the  carbohydrates  and  fats,  because  these  foodstuffs  are 
completely  oxidized  in  the  organism  into  carbon  dioxid  and 
water.  The  proteins,  on  the  other  hand,  are  not  fully 
reduced,  because  their  nitrogenous  end-product,  urea,  still 
contains  a  recognizable  amount  of  energy.  Their  full  value 
as  a  fuel,  therefore,  can  only  be  ascertained  if  the  amount  of 
energy  contained  in  the  urea  is  added  to  the  physiologically 
available  heat  derived  from  them.  It  has  been  determined 
that  1  gram  of  sugar  yields  4  calories,  and  1  gram  of  starch 
a  trifle  in  excess  of  this  figure.  Fat,  on  the  other  hand,  fur- 
nishes 9.3  calories  for  each  gram  of  substance  and  protein 
4.8  calories.  As  has  just  been  stated,  a  certain  amount  of 


276  NUTRITION 

the  energy  contained  in  the  latter  is  lost  with  the  urea, 
because  1  gram  of  protein  burned  in  the  open  yields  more 
than  6  calories.  Thus,  it  may  be  stated  that  the  aforesaid 
food  stuffs  give  the  following  results: 

1  gram  of  protein 4.1  calories 

1  gram  of  carbohydrates 4.1  calories 

1  gram  of  fat 9.3  calories 

The  Energy  Requirement  of  the  Body. — Heat  is  lost  not 
only  by  radiation  but  also  in  the  form  of  bound  heat. 
Hence,  a  person  living  in  a  calorimeter  furnishes  not  only 
a  certain  portion  of  heat  to  warm  up  the  water  in  the  appa- 
ratus, but  also  a  certain  amount  which  is  really  "latent," 
because  it  is  lost  through  the  evaporation  of  the  sweat. 
It  is  conceded  that  a  person  may  evaporate  about  one  liter 
(1000  grams)  of  water  in  the  course  of  twenty-four  hours. 
This  change  from  the  liquid  to  the  gaseous  state  requires 
heat  amounting  to  about  500  calories.  Obviously,  this 
correction  "must  be  made  when  the  total  heat  liberated  by 
the  person  is  to  be  computed. 

By  taking  all  these  factors  into  consideration,  it  has  been 
found  that  a  person,  when  sleeping,  requires  one  calorie  per 
hour  for  each  kilogram  of  weight.  Hence,  a  person  weighing 
70  kg.,  needs  about  1700  calories  to  balance  his  material 
metabolic  requirements.  The  consumption  of  energy  which 
just  covers  the  needs  of  a  resting  person,  is  known  as  basal 
consumption  or  basal  metabolism. 

The  heat  production  and  energy  requirement  of  a  person 
increases  considerably  when  work  is  done.  Muscular  ac- 
tivity is  the  chief  factor  to  be  considered  in  this  connection, 
because  even  during  comparative  rest  the  basal  metabolism 
rises  to  2100  calories,  and  reaches  the  value  of  2500  calories 
when  light  work  is  done.  Moderate  work  requires  3500 
calories  an^  heavy  work  4000  to  10,000  calories. 

The  Caloric  Value  of  Foods. — In  order  to  obtain  the  re- 
quired number  of  calories,  use  is  made  of  such  foods  as  are 
included  in  the  accompanying  table.  It  will  be  seen  that 
they  differ  greatly  in  their  chemical  composition  and  possess, 
therefore,  different  caloric  values. 


15 

11 

66 

4.2 

2.0 

1.7 

15 

12 

58 

5.4 

5.6 

3.0 

13 

6 

79 

0.4 

0.7 

0.5 

15 

23 

55 

2.0 

2.0 

2.0 

75 

2 

18 

3.0 

0.2 

0.7 

86 

4 

. 

24.0 

0.8 

37 

33 

3.0 

5.0 

72 

19 

29.0 

1.0 

51 

14 

1.0 

72 

18 

5.0 

1.0 

63 

16 

16.0 

1.0 

78 

18 

3.0 

1.0 

77 

16 

5.5 

1.5 

74 

14 

. 

105.0 

1.5 

15 

83.0 

3.0 

METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL    HEAT    277 

WATER    PROTEIN    STARCH    SUGAR  FAT       SALTS 

Bread 37  8  47         3.0  1.0         2.0 

Wheat  flour 

Oatmeal 

Rice 

Peas 

Potatoes 

Milk 

Cheese 

Lean  beef 

Fat  beef 

Mutton 

Veal 

White  fish 

Salmon 

Egg 

Butter 

As  a  rule  our  diet  is  arranged  to  yield  about  3000  calories. 
The  relative  proportions  of  the  three  principal  foodstuffs  are : 
75  grams  of  proteins,  90  grams  of  fats,  and  550  grams  of 
carbohydrates.  These  substances  furnish  the  following 
number  of  calories: 

Protein 70  grams  =    280  calories 

Fat 90  grams  =    810  calories 

Carbohydrates 550  grams  =  2200  calories 

3290  calories 

It  is  easily  apparent  that  the  carbohydrates  and  fats  bear  the 
brunt  of  the  metabolic  requirement,  while  protein  retains  a 
relatively  constant  substratum  value.  Hence,  any  additional 
demand  would  have  to  be  met  chiefly  by  a  greater  intake  of 
the  former  foodstuffs.  This  is  easily  explained,  because  a 
healthy  adult  person,  taking  a  fair  amount  of  exercise,  elimi- 
nates about  300  grams  of  carbon  and  20  grams  of  nitrogen, 
or  about  Y\  5  as  much  nitrogen  as  carbon.  In  order  to  derive 
this  quantity  of  carbon  from  meat,  about  1800  grams  or  4 
pounds  of  lean  beef  would  have  to  be  eaten,  while  more  than 
the  required  amount  of  nitrogen  can  be  obtained  from  453 
grams  or  1  pound  of  it.  Consequently,  a  person  who  derives 
his  fuel  chiefly  from  protein,  overtaxes  his  digestive  and  excre- 
tory organs,  and  follows,  moreover,  an.  unsound  economic 
path. 


278  NUTRITION 

For  this  reason,  a  mixed  diet  is  the  only  one  which  can  be 
justified  physiologically.  It  is  true,  however,  that  most 
foods  constitute  a  mixed  diet  in  themselves.  Thus,  meat 
contains  not  only  protein  substances,  but  also  from  30  to  50 
per  cent,  of  fat,  while  bread  embraces  the  protein,  glutin, 
as  well  as  starch,  sugar,  and  minute  quantities  of  fat.  Like- 
wise, milk  furnishes  water,  salts,  casein,  albumin,  emulsi- 
fied fat,  and  milk-sugar  or  lactose.  Milk,  however,  contains 
no  iron,  which  is  required  by  the  body  in  forming  the  hemo- 
globin of  the  red  blood  corpuscles. 

Conditions  of  life  may  be  such,  however,  that  a  well 
balanced  mixed  diet  cannot  be  maintained  for  long  periods  of 
time.  Thus,  we  find  that  the  Esquimaux  are  forced  to  live 
chiefly  upon  proteins  and  fats  and  must  derive  their  body- 
sugar  from  the  excess  protein.  It  has  been  mentioned 
above  that  the  surplus  protein  may  become  a  source  of  sugar 
and  fat.  The  other  extreme  is  presented  by  the  vegetarians 
who  reduce  their  ingestion  of  proteins  to  a  minimum,  al- 
though the  addition  of  milk  and  eggs  to  their  diet  would 
raise  it  practically  to  the  standard  of  a  mixed  diet.  Nuts 
and  vegetable  oils  furnish  their  requirement  in  fat. 

The  Body-temperature. — Every  cell  in  our  body  is  a  pro- 
ducer of  heat.  Admittedly,  however,  tissues  differ  greatly  in 
their  activities  and  hence,  also  in  their  power  of  liberating 
heat.  No  doubt,  the  most  important  heat-generating  organ 
is  the  skeletal  muscle  tissue,  because  we  well  know  that  even 
a  very  moderate  form  of  muscular  exercise  quickly  raises 
our  body-temperature  two  or  three  degrees. 

The  heat  liberated  by  the  cells  of  the  different  tissues,  is 
transferred  in  largest  part  to  the  blood  which  again  transfers 
it  to  the  air.  Thus,  it  has  been  found  that  the  average 
temperature  of  the  blood  traversing  internal  channels  is 
39°  to  40°  C.,  while  that  of  the  blood  in  the  more  exposed 
vessels  is  only  28°  to  35°  C.  It  will  be  seen,  therefore,  that 
the  production  of  heat  is  balanced  by  a  definite  loss,  and 
that  the  product  of  the  interaction  between  these  two  factors 
represents  the  temperature  of  the  body.  It  is  common 
knowledge  that  the  body-temperature  retains  a  relatively 
constant  value  in  some  animals  and  an  inconstant  value  in 


METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL    HEAT     279 

others.  The  former  are  commonly  designated  as  warm- 
blooded and  the  latter,  as  cold-blooded  animals. 

This  difference  suggests  that  the  warm-blooded  animals 
are  in  possession  of  a  mechanism  by  means  of  which  the  heat 
of  their  body  is  stagnated  in  amounts  just  sufficient  to  give 
them  a  practically  uniform  temperature.  Contrariwise,  the 
cold-blooded  animals  must  lose  their  heat  almost  as  rapidly  as 
it  is  formed,  because  they  have  no  means  of  regulating  its 
escape.  This  statement  leads  us  to  infer  that  their  body- 
temperature  must  be  equal  to  that  of  the  surrounding 
medium,  although  it  can  never  quite  reach  the  latter  value, 
because  the  metabolism  of  even  the  most  quiescent  animal 
cannot  be  made  to  cease  altogether.  For  this  reason,  a 
frog  living  in  water  of  20°  C.,  shows  a  body-temperature  of 
about  21°  C.  Warming  the  water  to  30°  C.  raises  its  body- 
temperature  to  about  31°  C.,  while  cooling  it  would  produce 
the  opposite  effect. 

In  this  connection,  it  should  also  be  remembered  that  the 
body-temperature  of  even  the  warm-blooded  animals  may 
be  varied  by  outside  influences,  but  usually  only  within 
tenths  of  one  degree.  Thus,  our  body-temperature  reaches 
its  lowest  point  early  in  the  morning  and  its  highest  level  late 
in  the  afternoon.  These  changes,  however,  rarely  amount 
to  more  than  1.2°  C.,  and  fluctuate  around  the  normal  value 
of  37°  C.  or  98.4°  F.  The  body-temperature  is  usually 
ascertained  by  placing  a  clinical  thermometer  under  the 
tongue,  meanwhile  protecting  it  against  radiation  by  closing 
the  lips.  When  measured  in  the  rectum,  the  temperature  is 
somewhat  higher,  namely,  37.3°  C.  Children  possess  a 
higher  body-temperature  than  adults,  because  they  are  more 
active. 

The  Regulation  of  the  Body-temperature. — A  uniform 
body-temperature  can  only  be  obtained  if  the  production 
and  dissipation  of  heat  are  accurately  balanced.  Thus,  a 
rise  in  the  body-temperature  may  be  due  either  to  a  greater 
heat  production  or  a  diminished  dissipation,  or  both.  Heat 
production  may  easily  be  reduced  by  muscular  rest,  and 
increased  by  exercise.  The  same  results  follow  the  varying 
activity  of  the  glandular  structures. 


280  NUTRITION 

Heat  dissipation  is  regulated  in  two  ways:  namely,  voli- 
ti anally  and  non-volitionally  by  reflex  action.  Thus,  the 
dwellings  of  man  are  constructed  to  resist  changes  in  the 
temperature  of  the  air,  and  different  clothing  is  worn  to 
correspond  to  the  time  of  day  and  the  seasons  of  the  year. 
Besides,  the  body  is  in  possession  of  several  mechanisms 
by  means  of  which  the  loss  of  heat  may  be  controlled  in  a 
reflex  manner.  Chiefly  concerned  in  this  regulation  are 
those  nervous  parts  which  vary  the  caliber  of  the  blood- 
vessels and  the  activity  of  the  glands.  It.  is  easily  con- 
ceivable that  the  dilatation  of  the  bloodvessels  of  the  skin 
must  favor  heat  dissipation,  because  a  larger  amount  of 
blood  is  then  exposed  to  the  neighboring  air.  Contrariwise, 
the  constriction  of  the  cutaneous  vessels  must  tend  to  con- 
serve the  heat,  because  the  vessels  of  the  skin  are  thereby 
rendered  relatively  bloodless.  In  this  regard,  therefore, 
the  bloodvessels  may  be  likened  to  the  pipes  of  a  heating 
apparatus.  The  heat  imparted  to  their  contents  by  innumer- 
able minute  furnaces  situated  in  the  cells  of  the  tissues,  is 
transferred  by  them  in  part  to  the  air. 

A  very  important  factor  concerned  in  this  process  of  regu- 
lating the  body-temperature,  is  the  sweat.  If  the  skin  is 
thoroughly  moistened  with  this  secretion,  a  much  greater 
amount  of  heat  will  be  lost  than  when  it  is  relatively  dry. 
Ordinarily,  the  dilatation  of  the  cutaneous  vessels  is  associ- 
ated with  a  more  copious  production  of  sweat,  both  changes 
becoming  more  intense  when  a  greater  heat — dissipation  is  to 
be  effected.  Contrariwise,  the  constriction  of  the  blood- 
vessels of  the  integument  usually  diminishes  the  quantity  of 
this  secretion,  because  this  general  reaction  reduces  in  most 
instances  also  the  bloodsupply  of  the  sweat  glands.  A  simi- 
lar function  is  performed  by  the  water  moistening  the  lining 
of  the  respiratory  passage.  Thus,  those  animals  whose 
integument  is  covered  with  a  thick  coat  of  hairs,  a  pnpr 
conductor  of  heat,  rid  themselves  of  their  superfluous  heat 
by  the  act  of  panting.  The  respiratory  air  is  then  forced  in 
quickly  repeated  draughts  across  the  moistened  mucous 
membranes  of  the  mouth  and  upper  air-passage  so  as  to 
increase  evaporation. 


METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL   HEAT     281 

A  similar  means  of  heat  dissipation  is  resorted  to  by  us 
during  muscular  exercise.  It  consists  in  increasing  the 
frequency  and  depth  of  the  respiratory  movements.  While 
the  primary  purpose  of  this  greater  ventilation  of  the  lungs  is 
the  augmentation  of  the  gas  exchange,  it  also  facilitates  the 
discharge  of  heat  through  this  channel  by  radiation  as  well 
as  by  the  evaporation  of  the  moisture.  When  exercising, 
the  body  is  constantly  brought  in  contact  with  fresh  layers  of 
air  which  have  not  been  warmed  up  as  yet.  Thus,  slight 
movements  afford  the  same  relief  as  an  electric  fan,  because 
they  bring  the  surface  of  our  skin  in  relation  with  cooler  air. 

The  foregoing  discussion  leads  us  to  infer  that  the  heat  of 
the  body  is  dissipated  directly  by  radiation  and  conduction, 
as  well  as  indirectly  by  evaporation.  By  radiation  is  meant 
the  transfer  of  the  heat  to  another  body  through  space  with- 
out causing  an  appreciable  change  in  the  temperature  of  the 
air.  But,  heat  may  also  be  conducted  from  one  body  to 
another  by  contact.  Thus,  if  the  hand  is  placed  against  a 
cold  window  pane,  it  gives  off  heat  to  the  glass,  while  the 
latter  in  turn  loses  it  to  the  adjoining  layers  of  air.  Quite 
similarly,  a  person  immersed  in  cold  water,  transfers  his 
surplus  heat  to  the  medium  which  in  turn  warms  up  the  walls 
of  the  receptacle  and  adjoining  air.  Heat  is  also  lost  through 
the  medium  of  the  water  discharged  by  the  body,  because  the 
evaporation  of  every  globule  of  it  necessitates  the  expenditure 
of  a  certain  portion  of  this  energy. 

Attention  has  already  been  called  to  the  fact  that  humid 
and  warm  air  produces  a  certain  discomfort,  because  it 
prevents  the  dissipation  of  heat  by  lessening  evaporation. 
The  sweat  then  accumulates  upon  the  surface  of  the  body  in 
visible  drops,  each  of  them  retaining  a  certain  amount  of 
bound  heat.  A  dry  atmosphere,  on  the  other  hand,  favors 
evaporation.  The  skin  then  becomes  relatively  dry,  al- 
though the  amount  of  water  actually  lost  by  it  may  be  greater 
than  in  the  former  instance.  Thus,  the  stoker  in  the  engine 
room  is  able  to  toil  in  a  hot  atmosphere,  because  he  secretes 
a  very  copious  quantity  of  sweat  which  is  quickly  taken  up 
by  the  heated  air.  A  person  working  in  a  passage  under 
ground,  might  really  be  in  a  less  favorable  situation  than  the 


282  NUTRITION 

stoker,  because  while  the  air  is  cooler  here,  its  greater  relative 
humidity  retards  the  loss  of  bound  heat. 
^  Fever. — Temporary  rises  of  the  body-temperature  above 
normal  are  produced  most  easily  by  muscular  exercise.  In 
this  case,  the  heat-production  is  augmented  so  rapidly  that  it 
cannot  be  compensated  for  completely  until  several  minutes 
thereafter.  Rises  of  a  more  permanent  kind  may  be  evoked 
by  working  in  a  humid  and  warm  atmosphere,  because  in 
this  instance  the  greater  production  of  heat  is  antagonized 
by  a  lessened  dissipation.  It  is  a  well  known  fact  that  the 
entrance  into  the  body  of  certain  disease  producing  bacteria 
gives  rise  to  a  rather  prolonged  elevation  of  the  body-tem- 
perature which  is  commonly  designated  as  fever.  As  in  the 
above  instances,  this  condition  must  be  brought  about  by  a 
disproportionate  relationship  between  heat-production  and 
heat-dissipation,  leaving  a  positive  balance  for  the  body- 
temperature. 

The  chief  factor,  however,  seems  to  be  a  disturbance  of  the 
dissipating  mechanism  which  produces  a  stagnation  of  heat 
even  at  a  time  when  its  production  remains  practically  the 
same.  A  sensation  of  chilliness  is  usually  experienced  at  the 
beginning  of  a  fever,  because  the  superficial  bloodvessels 
then  constrict,  causing  in  turn  a  cessation  of  the  secretion  of 
sweat.  As  the  skin  cools,  certain  reflexes  are  evoked,  the 
purpose  of  which  is  to  counteract  the  fall  in  the  body-tem- 
perature. Even  the  smooth  muscles  of  the  skin  contract  in 
an  endeavor  to  prevent  this  loss  by  pressing  upon  the  cuta- 
neous vessels.  The  regions  of  the  roots  of  the  hairs  and 
pores  are  thereby  made  to  project  from  the  surface  in  the 
form  of  minute  papillae,  giving  rise  to  an  appearance  of  the 
skin  which  is  generally  known  as  goose-flesh.  The  muscles 
are  xnade  to  quiver  in  an  endeaver  to  produce  a  surplus 
amount  of  heat  in  order  to  overcome  the  sensation  of  chilli- 
ness. The  temperature  now  rises  very  rapidly,  while  the 
patient  appeals  for  heavier  covers.  The  skin  becomes 
flushed  and  feels  distinctly  hot. 

Later  on  in  the  course  of  the  fever,  the  production  of  heat 
is  materially  diminished,  but  since  the  constriction  of  the 
cutaneous  vessels  continues,  its  dissipation  must  still  be  too 


METABOLIC    REQUIREMENTS    OF   BODY    ANIMAL    HEAT     283 

slight  to  cause  a  decided  lowering  of  the  body -temperature. 
The  "  breaking-point "  of  the  fever  is  indicated  by  a  gradual 
return  of  the  body-temperature  to  normal.  The  skin  then 
loses  its  livid,  flushed  appearance  and  is  again  moistened 
with  sweat.  Large  amounts  of  urine  are  secreted,  its  color 
gradually  becoming  lighter.  The  activity  of  the  heart  and 
respiratory  parts  decreases  constantly  until  normal  condi- 
tions have  again  been  established. 

Fever  is  not  altogether  a  pathological  process,  but  con- 
stitutes really  a  physiological  reaction  of  the  cells  to  bacteria 
and  their  products.  In  fact,  it  may  also  be  a  protective 
means,  because  many  bacteria  are  killed  at  a  temperature  of 
38°  to  40°  C.  It  is  true,  however,  that  the  increased  produc- 
tion of  heat  is  associated  with  a  disturbance  in  the  dissipation 
of  the  body-heat,  which  is  brought  about  chiefly  by  the  con- 
striction of  the  cutaneous  vessels  and  the  cessation  of  the 
secretion  of  sweat.  During  this  period  of  increased  metabo- 
lism, the  cells  oxidize  principally  the  stored  proteins  and 
not  the  fats  and  carbohydrates. 


CHAPTER  XXVIII 
EXCRETION 

The  Excretory  Channels. — The  term  excretion  is  commonly 
applied  to  that  process  which  purposes  to  remove  the  waste 
matter  from  the  body.  In  its  passage  through  the  different 
tissues,  the  blood  is  incessantly  loaded  with  the  products  of 
cellular  decomposition.  It  follows  that  if  this  medium  is  to 
be  kept  in  a  proper  functional  condition,  these  waste  sub- 
stances must  be  removed  from  it  almost  as  quickly  as  they 
are  formed.  The  organs  which  are  directly  concerned  with 
this  elimination,  are  the  lungs,  kidneys,  skin,  and  alimentary 
canal.  The  chief  gaseous  excretion  is  furnished  by  the  lungs 
in  the  form  of  carbon  dioxid.  It  constitutes  the  final  product 
of  the  metabolism  of  the  carbon  of  the  food.  Water  is 
discharged  by  the  kidneys,  skin,  intestinal  canal,  and  the 
mucous  lining  of  the  pulmonary  passage.  The  excretion  of 
the  kidneys  also  embraces  the  various  end-products  of 
protein  metabolism,  such  as  urea,  creatin  and  creatinin. 

The  waste  from  the  intestine  is  of  miscellaneous  character, 
and  finds  its  origin  in  the  excrements  of  the  bile.  The  loss  of 
water  by  this  route  is  inconsiderable  under  normal  conditions. 
The  discharge  of  carbon  dioxid  has  already  been  discussed 
in  detail  in  a  preceding  chapter,  so  that  we  are  now  able  to 
direct  our  attention  more  particularly  to  the  excretory  func- 
tions of  the  kidneys  and  skin. 

The  Kidneys.— The  kidneys  are  situated  in  the  lumbar 
regions  of  the  abdominal  cavity,  one  on  each  side  of  the  spinal 
column.  Each  organ  measures  about  100  mm.  in  length, 
50  mm.  in  breadth,  and  25  mm.  in  thickness.  It  occupies 
the  space  between  the  upper  borders  of  the  12th  thoracic  and 
third  lumbar  vertebrae.  Its  shape  is  similar  to  that  of  a 
bean,  its  concave  side  being  directed  toward  the  spinal  col- 
umn. Near  the  center  of  this  concavity  is  a  decided  depres 

284 


EXCRETION 


285 


sion,  known  as  the  hilum,  through  which  the  bloodvessels 
and  nerves  enter  this  organ.  The  channel  supplying  it  with 
nutritive  material  is  called  the  renal  artery.  It  arises  directly 
from  the  abdominal  aorta.  The  venous  drainage  of  this 
organ  is  conveyed  by  the  renal  vein  into  the  inferior  vena 
cava.  In  addition,  the  hilum  also  serves  as  the  point  of 


FIG.  108. — Median  longitudinal  section  through  the  right  kidney.  A,  A, 
calyces  minores;  B,  B,  calyces  majores;  C,  pelvis  of  the  ureter;  S,  hilus  of 
the  kidney.  (Radasch.) 

exit  for  the  ureter,  a  musculo-membranous  tube  by  means 
of  which  the  secretion  of  the  kidney  is  conveyed  into  the 
urinary  bladder. 

Each  kidney  rests  upon  a  cushion  of  adipose  tissue,  and  is 
invested  by  a  dense  capsule  of  connective  tissue.  When  cut 
across  longitudinally,  it  presents  three  distinct  zones :  namely, 
an  outer  one  or  cortex,  a  middle  one  or  medulla,  and  an  inner 
one  or  pelvis.  This  structural  peculiarity  is  dependent  upon 
the  manner  in  which  the  functional  element  of  this  organ  is 
arranged.  It  is  to  be  noted  first  that  each  kidney  is  com- 
posed of  a  very  large  number  of  tubular  glands,  which  begin 
in  its  cortical  substance  and  terminate  at  the  junction 


286  NUTRITION 

between  its  medullary  portion  and  the  pelvis.  This  inner 
membranous  receptacle  of  this  organ  really  represents  the 
enlarged  upper  extremity  of  the  ureter. 

The  Uriniferous  Tubule. — The  tubular  glands  of  which 
each  kidney  is  composed,  are  called  uriniferous  tubules. 
They  are  concerned  with  the  secretion  of  the  urine.  Each 
tubule  furnishes  a  small  globule  of  this  excretion  which  is 
conveyed  into  the  pelvis,  where  it  is  added  to  those  secreted 


FIG.  109. — Glomerulus  with  the  beginning  segment  of  the  uriniferous 
tubule.  G,  glomerulus;  A  and  E,  afferent  and  efferent  blood-vessels;  C, 
capsule  of  Bowman;  .V,  neck  of  uriniferous  tubule;  CT,  distal  convoluted 
tubule. 

by  the  other  tubules.  It  is  easily  understood  that  the 
constant  influx  of  urine  from  the  different  glands  must  finally 
lead  to  a  distention  of  the  walls  of  this  reservoir  and  evoke 
certain  reflexes  which  cause  its  muscle  fibers  to  contract. 
The  purpose  of  this  contraction  is  to  force  the  contents  of  the 
pelvic  reservoir  into  the  ureter  proper,  whence  they  are  pro- 
pelled into  the  bladder.  The  movements  of  the  ureter  bear 
a  close  resemblance  to  those  previously  observed  in  the  oeso- 
phagus, stomach,  and  intestines.  They  are  peristaltic  in 
character,  and  invariably  pursue  a  course  from  above 
downward.  These  waves  recur  at  brief  intervals,  every  one 
of  them  forcing  a  droplet  of  urine  into  the  bladder. 


EXCRETION 


287 


Each  urinary  tubule  begins  in  the  cortex  (bark)  of  the 
kidney  in  a  rounded  dilatation  which  is  known  as  the  capsule 


FIG.  110. — Diagrammatic  representation  of  the  blood-supply  and 
course  of  the  uriniferous  tubule.  J,  interlobular  blood-vessels  derived 
from  arches  between  cortex  and  medulla ;  G,  glomeruli ;  C,  distal  convoluted 
tubule;  D  and  A,  descending  and  ascending  limbs  of  the  loop  of  Henle; 
CT,  collecting  tubule ;  P,  papilla  and  pelvis  of  the  kidney. 

of  Bowman.     A  coil  of  capillaries  is  freely  suspended  within 
this  capsule  in  such  a  way  that  it  forms  contact  with  the 


288  NUTRITION 

wall  of  this  cavity  only  at  the  point  where  the  arterial 
supply  channel  enters.  The  term  glomerulus  or  Malpighian 
corpuscle  is  usually  applied  to  this  entire  structure.  After 
leaving  the  capsule,  each  tubule  pursues  a  serpentine  course 
and  is  then  reflected  upon  itself  to  form  a  U-shaped  segment 
which  is  called  the  loop  of  Henle.  Distally  to  this  point,  the 
tubule  again  describes  several  irregular  curves,  and  finally 
unites  with  others  into  a  collecting  channel  which  opens  into 
the  pelvis. 

In  general,  therefore,  it  may  be  said  that  each  tubule 
embraces  a  central  and  a  distal  convoluted  portion,  these  seg- 
ments being  separated  from  one  another  by  the  loop  of  Henle. 
Because  of  the  fact  that  the  collecting  tubules  and  loops  of 
Henle  strive  in  a  radial  direction  towards  the  pelvis,  while  the 
glomeruli  and  convoluted  segments  occupy  the  outer  region 
of  this  organ,  its  substance  seems  to  be  composed  of  two 
distinct  layers  which,  as  has  been  stated  above,  are  designated 
as  its  cortex  and  medulla.  This  peculiar  distribution  of  the 
urinary  tubule  also  accounts  for  the  striated  appearance  of 
its  medullary  portion.  Since  many  collecting  tubules  are 
always  bound  together  into  bundles,  each  group  of  which 
possesses  a  single  orifice,  the  medulla  appears  in  the  form  of  a 
number  of  fan-like  structures  which  are  known  as  the 
pyramids. 

The  Secretion  of  Urine. — The  general  structure  of  the 
glomerulus  reminds  us  of  that  of  an  ordinary  filter,  in  which 
the  paper  is  represented  by  the  walls  of  the  blood-capillaries. 
The  upper  surface  of  this  membrane  is  exposed  to  the 
pressure  ordinarily  prevailing  in  the  capillaries  and  amount- 
ing to  about  40  mm.  Hg,  while  its  under  surface  lies  in  rela- 
tion with  the  free  space  of  the  capsule  of  Bowman  in  which 
the  pressure  is  practically  zero.  In  accordance  with  this 
structural  peculiarity  it  was  assumed  at  an  early  date  that 
the  secretion  of  urine  is  entirely  dependent  upon  the  physical 
factor  of  differences  in  pressure.  This  assumption  justified 
the  early  view  that  the  capillary  lining  permits  every  con- 
stituent of  the  blood  to  pass  through,  excepting,  of  course, 
the  formed  elements.  In  accordance  with  this  conception,  the 
glomerulus  was  supposed  to  yield  urine  in  its  complete  form. 


EXCRETION  289 

Repeated  experimentation,  however,  has  proved  that  the 
secretion  of  urine  is  by  no  means  a  process  of  simple  filtration, 
because  the  capillary  lining  of  the  glomerulus  lets  certain 
constituents  of  the  blood  pass  through  freely,  while  it  prevents 
the  escape  of  other  very  similar  substances.  In  order  to 
be  able  to  explain  this  discrepancy,  the  factors  of  diffusion 
and  osmosis  were  added  to  that  of  filtration.  In  further 
analysis  of  this  subject-matter  it  was  then  observed  that 
while  the  three  factors  just  enumerated  play  a  very  important 
part  in  the  formation  of  urine,  the  cells  of  the  urinary  tubule 
no  doubt  exert  a  decided  influence  upon  the  character  of  this 
secretion.  In  other  words,  there  has  been  added  to  the 
three  aforesaid  factors  a  fourth  one,  embracing  the  minute 
chemico-physical  properties  or  vital  activity  of  the  cells  lining 
the  urinary  tubule.  Thus,  it  has  finally  been  concluded 
that  the  water  and  inorganic  salts  of  the  urine  are  formed  in 
the  glomeruli,  while  the  cells  of  the  adjacent  convoluted 
tubules  add  to  this  secretion  its  organic  constituents.  Hence, 
it  may  be  said  that  the  urine  becomes  more  concentrated 
as  it  traverses  the  convoluted  tubule. 

The  Composition  of  Urine. — The  renal  secretion  is  a  clear 
fluid,  possessing  a  light  straw-color  and  a  specific  gravity  of 
about  1.020.  Its  reaction,  which  is  moderately  acid,  is  due 
to  the  presence  of  acid  sodium  phosphate.  On  standing 
the  urine  becomes  alkaline,  because  its  chief  constituent 
urea  is  then  decomposed  under  production  of  ammonium 
carbonate.  It  then  emits  an  ammoniacal  odor  and  becomes 
cloudy. 

A  healthy  person  secretes  about  1500  c.c.  (50  ounces  or 
2j/£  pints)  of  urine  in  the  course  of  twenty-four  hours.  Its 
quantity,  however,  may  be  varied  considerably  in  different 
ways:  for  example,  by  increasing  or  decreasing  the  intake  of 
water  and  by  altering  the  activity  of  the  sweat  glands.  Inas- 
much as  a  direct  relationship  exists  between  these  excretory 
organs,  it  is  evident  that  a  more  copious  secretion  of  sweat 
must  diminish  the  amount  of  urine.  The  latter  then  becomes 
much  darker  in  color  and  more  concentrated. 

The  urine  contains:  (a)  inorganic  material  in  the  form  of 
sodium  chlorid  and  the  sulphates  and  phosphates  of  sodium, 

19 


290  NUTRITION 

potassium,  calcium  and  magnesium;  (b)  organic  material, 
chiefly  in  the  form  of  urea;  (c)  coloring  material  which  is 
derived  from  the  reduced  pigments  of  the  bile;  and  (d) 
traces  of  certain  gases,  such  as  carbon  dioxid,  nitrogen,  and 
oxygen.  The  sodium  chlorid  possesses  no  metabolic 
history,  and  simply  leaves  the  body  through  this  channel  in 
amounts  proportionate  to  those  ingested.  The  sulphates 
and  phosphates,  on  the  other  hand,  are  derivatives  of 
the  proteins  which  leave  the  body  in  these  particular 
combinations. 

Urea  is  not  made  in  the  kidneys,  but  is  merely  picked  out 
by  these  organs  from  the  blood  and  transferred  into  the 
lumen  of  the  urinary  tubule.  It  is  formed  in  the  liver  from 
the  protein  waste  of  different  tissues,  principally  that  of  the 
muscles,  and  also  from  the  superfluous  nitrogen  of  the  food. 
A  normal  adult  eliminates  about  30  grams  of  this  substance 
in  the  course  of  a  day,  but  this  quantity  may  be  greatly 
increased  by  muscular  exercise  as  well  as  by  a  surplus  inges- 
tion  of  proteins.  Inasmuch  as  the  protein  content  of  the 
body  remains  relatively  stationary,  nothing  can  be  gained 
by  consuming  excessive  amounts  of  this  foodstuff.  In 
fact,  the  only  result  would  be  an  overtaxing  of  renal  function 
which  would  become  the  more  dangerous  when  only  small 
amounts  of  water  are  taken.  It  has  been  stated  elsewhere 
that  urea  represents  the  nitrogenous  product  of  the  cleavage 
of  the  protein  molecule,  although  a  portion  of  it,  namely, 
one-eighth  of  the  total,  is  transformed  into  other  more 
complex  constituents,  such  as  uric  acid  and  creatin. 

The  urine  also  contains  traces  of  sugar  and  albumin. 
Whenever  these  constituents  appear  in  appreciable  amounts, 
they  are  indicative  of  a  disturbance  in  renal  function.  A 
temporary  glycosuria,  as  we  have  seen,  may  be  perfectly 
physiological,  because  it  merely  serves  to  eliminate  surplus 
amounts  of  sugar.  Likewise,  a  temporary  albuminuria 
may  follow  severe  muscular  exercise.  The  urine  also  con- 
tains traces  of  indican,  a  constituent  derived  from  the  putre- 
faction of  the  protein  food  in  the  large  intestine.  Excessive 
amounts  of  this  substance  would  suggest  a  corresponding 
increase  in  the  putrefaction  of  this  foodstuff.  A  similar 


EXCRETION  291 

body  is  acetone.  Its  origin  is  to  be  sought  in  an  incomplete 
oxidation  of  the  fats  and  possibly  also  of  the  proteins. 

The  Storage  of  Urine. — The  purpose  of  the  successive 
peristaltic  waves  descending  along  the  ureter,  is  to  transfer 
the  contents  of  the  pelvis  at  intervals  into  the  urinary  blad- 
der. The  latter  presents  itself  as  an  oval  musculo-membran- 
ous  pouch  which  is  lined  internally  by  mucous  membrane 
and  externally  by  peritoneum.  It  is  situated  in  the  pelvis 
behind  the  pubic  bone,  its  size  and  shape  varying  somewhat 
with  the  amount  of  urine  contained  therein.  The  ureters 
pierce  its  posterior  and  inferior  wall  at  some  distance  from 
one  another  and  in  a  slanting  direction.  This  arrangement 
serves  the  purpose  of  a  sphincter,  because  the  gradual  dis- 
tention  of  the  bladder  gives  rise  to  a  compression  of  these 
orifices.  The  lower  pole  of  the  bladder  gradually  tapers  into 
a  narrow'  membranous  canal,  the  urethra,  which  opens  to  the 
outside  by  an  orifice,  known  as  the  urinary  meatus.  The 
point  of  union  between  this  canal  and  the  tapering  end  or 
neck  of  the  bladder  is  marked  by  two  conspicuous  bands  of 
circular  muscle  tissue  which  are  termed  the  internal  and 
external  sphincters  of  the  urethra.  These  fibers  are  normally 
held  in  a  condition  of  tonus,  so  that  the  urethra  is  kept  free 
from  urine  during  the  interims  between  the  acts  of  micturition. 

When  empty,  the  urinary  bladder  presents  a  shriveled  up 
appearance.  For  this  reason,  the  inflow  of  urine  through  the 
ureters  cannot  at  first  establish  a  considerable  pressure 
within  its  cavity.  Later  on,  however,  when  its  walls  have 
become  somewhat  distended,  the  pressure  rises  more  rapidly, 
reaching  its  maximal  value  of  about  150  mm.  H2O  at  a 
time  when  about  250  c.c.  of  urine  have  been  collected  therein. 

The  voiding  of  the  urine  constitutes  the  act  of  micturition. 
The  factors  involved  in  it  are  the  contraction  of  the  walls  of 
the  bladder  and  the  relaxation  of  the  sphincters  of  the 
urethra.  An  additional  rise  in  pressure  may  be  produced  by 
the  contraction  of  the  abdominal  musculature.  The  local 
motor  mechanism  of  the  bladder  is  under  the  control  of  a 
simple  reflex  center  which  is  situated  in  the  lumbar  segment  of 
the  spinal  cord.  Its  activation  is  accomplished  by  the  differ- 
ent impulses  generated  in  consequence  of  the  distention  of 


292 


NUTBITION 


the  walls  of  the  bladder.  It  should  be  remembered,  how- 
ever, that  in  the  adult  mammal  the  opening  of  the  sphincters 
and  contraction  of  the  musculature  of  this  organ  may  be 
inhibited  for  a  time  by  the  higher  cerebral  centers. 

The  Skin  as  an  Excretory  Organ. — The  skin  consists  of 

an  outer  layer  or  epidermis  and 
an  inner  layer  or  dermis.  Below 
the  latter  lies  a  loose  reticulum 
of  fibrous  connective  tissue 
which  is  often  greatly  infiltrated 
with  fat  and  forms  the  so-called 
panniculus  adiposus.  The  epi- 
dermis is  composed  of  many 
layers  of  flat  cells,  the  outermost 
of  which  are  constantly  lost  and 
replaced  by  those  occupying 
the  next  stratum.  Those  of  the 
deepest  layer  possess  a  certain 
thickness  and  multiply  by  cell- 
division,  whereupon  they  move 
outward  toward  the  surface  of 
the  body.  Since  the  epidermis 
is  devoid  of  bloodvessels,  these 
cells  undergo  retrogressive 
changes  and  are  finally  con- 
verted into  flat,  scale-like 
platelets. 

The  deep  skin  or  dermis  also 
consists  of  two  zones:  namely, 
a  superficial  or  papillary  layer 
and  an  inner  or  reticular  layer. 
The  dermis  is  made  up  of  a 
dense  network  of  connective 
tissue  fibers  in  which  are  im- 
bedded many  elastic  fibers.  It  also  embraces  many  sensory 
corpuscles  and  glands.  The  former  are  modified  end-organs 
of  nerves,  and  subserve  the  sensations  of  touch,  pain,  and 
temperature.  The  glands  of  the  skin  appear  in  two  forms: 
namely,  as  sweat  glands  and  as  sebaceous  glands.  Their 


FIG.  111. — Diagrammatic 
representation  of  the  skin,  show- 
ing the  location  of  the  sweat 
glands.  H,  horny  layer;  L, 
stratum  lucidum ;  M ,  Malpighian 
layer;  P,  corpuscles  of  Paccini; 
PL,  papillae  of  the  cutis  vera; 
C,  cutis  vera;  S,  sweat  gland; 
SC,  subcutaneous  tissue. 


EXCRETION  293 

number  varies  considerably  in  different  areas  of  the  body. 
Thus,  the  sweat-glands  are  very  numerous  upon  the  palms 
of  the  hands  and  the  soles  of  the  feet.  As  many  as  two  or 
three  thousand  may  be  found  here  in  a  square  inch  of  skin. 

Each  sweat-gland  consists  of  a  coiled-up  portion  and  a 
duct.  The  latter  possesses  a  length  of  about  0.8  cm.  (one- 
fourth  of  an  inch),  and  pursues  a  serpentine  course  through 
the  outer  layers  of  the  skin,  opening  finally  by  means  of  a 
pore  upon  its  surface.  The  coil  is  surrounded  by  a  dense 
network  of  capillaries.  The  secretion  furnished  by  these 
glands  is  very  watery,  containing  a  small  amount  of  salts, 
fatty  acids,  carbon  dioxid,  and  traces  of  urea.  Its  content 
in  urea,  however,  may  be  markedly  increased  when  the  kid- 
neys are  not  in  a  proper  functional  condition.  For  this 
reason,  persons  are  made  to  produce  copious  amounts  of 
sweat  whenever  they  show  an  intoxication  in  consequence  of 
the  accumulation  of  the  products  of  protein  metabolism. 
Attention  has  already  been  called  to  the  close  relationship 
existing  between  the  quantity  of  the  urine  and  that  of  the 
sweat.  Obviously,  if  considerable  amounts  of  water  are 
lost  through  the  sweat-glands,  the  kidneys  cannot  eliminate 
their  normal  quantity  of  urine.  The  functions  of  both  organs 
may  in  turn  be  influenced  by  the  escape  of  water  into  the 
large  intestine.  Thus,  watery  stools  invariably  diminish 
the  secretion  of  .urine  as  well  as  that  of  sweat. 

Under  normal  circumstances,  the  sweat  is  evaporated  from 
the  surface  of  the  skin  almost  as  quickly  as  it  is  formed.  It 
is  then  called  insensible  perspiration.  But  when  the  body  is 
exposed  to  a  warm  and  humid  atmosphere,  or  when  a  greater 
amount  of  sweat  is  produced  than  can  readily  be  evaporated, 
it  accumulates  upon  the  surface  in  the  form  of  drops.  We 
then  speak  of  it  as  sensible  perspiration.  Under  average 
conditions,  close  to  one  liter  of  sweat  is  excreted  in  the 
course  of  twenty-four  hours.  This  quantity  may  be  greatly 
increased  by  exercise. 

The  sebaceous  glands  are  small  globular  glands  which 
usually  lie  in  relation  with  the  roots  of  the  hairs,  but  may  also 
occur  independently  of  these  appendages.  They  furnish 
an  oily  secretion,  containing  fats,  epithelial  cells,  inorganic 


294  NUTRITION 

salts,  and  albuminous  matter.  Its  principal  purpose  is  to 
form  a  protective  layer  on  the  surface  of  the  skin,  thereby 
keeping  the  integument  and  the  hairs  in  a  soft  and  pliable 
condition.  The  glands  situated  in  the  skin  of  the  nose  and 
forehead,  are  usually  very  large,  and  their  excretory  ducts 
are  frequently  plugged  with  sebaceous  material  that  furnishes 
a  fertile  soil  for  the  growth  of  the  ordinary  pus-microbes. 
It  is  also  of  interest  to  note  that  the  sebaceous  glands  of  the 
external  auditory  canal  furnish  a  modified  secretion  which 
contains  a  yellowish  pigment  and  hardens  on  exposure  to  the 
air.  This  waxy  material  is  called  cerumen.  On  occasions 
it  may  form  a  solid  mass  of  material  which  completely 
blocks  the  entrance  to  the  middle  ear,  thereby  greatly  im- 
pairing the  vibration  of  the  ear  drum  and  diminishing  the 
acuity  of  hearing. 

The  skin  also  contains  numerous  smooth  muscle  cells 
which  usually  lie  in  relation  with  the  roots  of  the  hairs. 
Their  contraction  is  effected  reflexly  in  consequence  of  emo- 
tions and  the  stimulation  of  the  integument  by  cold.  Inas- 
much as  these  cells  are  directed  almost  transversely  to  the 
roots  of  the  hairs,  their  contraction  must  cause  the  shafts  of 
these  appendages  to  assume  a  more  vertical  position.  This 
mechanism  is  brought  into  play  by  many  animals  as  a  means 
of  protection  against  their  natural  enemies.  Probably  the 
most  familiar  reactions  of  this  kind  are  the  erection  of  the 
hairs  upon  the  tail  and  dorsal  aspect  of  the  cat's  body,  and 
the  erection  of  the  bristle-like  appendages  and  spines  of  the 
porcupine. 


CHAPTER  XXIX 
THE  INTERNAL  SECRETIONS 

Classification  of  the  Endocrine  Organs. — The  subject- 
matter  of  the  internal  secretions  is  a  comparatively  recent 
one.  It  is  concerned  with  the  functions  of  those  glandular 
bodies  which  do  not  possess  a  recognizable  excretory  duct, 
and  pour  their  products  directly  into  the  blood  or  lymph. 
Usually  insignificant  in  size  and  tucked  away  in  out  of  the 
way  places  of  the  body,  these  endocrine  structures  failed  to 
attract  attention  until  about  the  year  1880,  when  it  was 
proved  that  the  total  removal  of  the  thyroids  for  the  relief 
of  the  disturbing  symptoms  associated  with  goiter  produced 
death  within  a  relatively  short  period  of  time.  Contrari- 
wise, it  was  noted  that  the  partial  removal  of  this  gland  did 
not  give  rise  to  any  untoward  effects.  More  recently,  these 
observations  have  been  repeated  upon  the  adrenal  bodies, 
pancreas,  and  other  organs  of  this  character.  Inasmuch  as 
the  removal  of  any  one  of  these  structures  by  operative 
means,  accidental  injuries,  or  pathological  processes  invari- 
ably results  in  dangerous  consequences  to  man  and  the 
mammals  in  general,  it  can  no  longer  be  doubted  that  these 
glands  exert  a  powerful  influence  upon  the  functions  of  the 
body. 

The  chemical  nature  of  the  products  of  the  endocrine 
organs  is  not  fully  known,  although  it  is  possible  to  employ 
them  in  an  experimental  way.  For  this  purpose  we  usually 
prepare  them  by  macerating  and  extracting  the  gland  as  a 
whole  in  a  solution  of  0.7  per  cent,  sodium  chlorid,  this 
extract  then  being  injected  directly  into  the  bloodstream 
to  see  what  effects  it  produces.  In  the  case  of  the  adrenal 
bodies,  the  active  principle  has  been  isolated  and  is  now  sold 
in  the  form  of  a  commercial  preparation,  known  as  adrenalin. 
A  somewhat  similar  "  purification "  has  been  attained  in  the 
case  of  the  active  agent  of  the  extract  of  thyroid  gland. 

295 


-296 


NUTRITION 


The  organs  belonging  to  the  group  of  the  endoerines 
(Greek:  within,  to  separate),  are  the  thyroids,  parathyroids, 
thymus,  adrenals  or  suprarenal  capsules,  pancreas,  liver, 
pineal  gland,  pituitary  body,  testes,  and  ovaries.  Every 
one  of  these  structures  furnishes  a  product  which  is  essential 
to  the  life  of  the  animal.  The  general  term  employed  to 
designate  these  drug-like  principles,  is  autacoid  substances 
(Greek:  remedy,  natural).  But  since 
they  may  accelerate  as  well  as  retard 
a  certain  function,  they  are  usually 
divided  into  two  groups,  embracing, 
on  the  one  hand,  the  hormones  (Greek : 
to  excite)  and,  on  the  other,  the  cha- 
lones  (Greek :  to  make  slack) .  A  typ- 
ical hormone  is  the  secretin  of  the 
duodenal  mucosa  which  stimulates  the 
flow  of  the  pancreatic  juice.  Among 
the  chalones  might  be  mentioned  the 
active  agent  of  the  adrenals  which  pre- 
vents the  excessive  mobilization  of  the 
glycogen  of  the  liver. 

The  Thyroid  Gland. — The  thyroid 
gland  consists  of  two  lobes  which  are 
connected  with  one  another  by  a  narrow 
bridge  of  tissue .  They  are  nearly  equal 
in  size  and  measure  about  5  cm.  in 
length.  Their  combined  weight  is  30 
grams.  In  man  they  assume  a  position 
in  close  relation  with  the  trachea,  at  its  junction  with  the 
cricoid  and  thyroid  cartilages  of  the  larynx.  Their  sub- 
stance is  composed  of  a  large  number  of  acini,  containing  in 
their  interior  a  viscous  colloid  material.  It  is  supposed  that 
this  material  finds  its  way  into  the  adjoining  lymph  chan- 
nels. It  is  also  of  importance  to  note  that  their  substance 
embraces  four  small  bodies,  the  cells  of  which  present  certain 
characteristics  which  clearly  differentiate  them  from  the 
surrounding  mass  of  the  thyroids.  These  structures  form 
the  so-called  parathyroid  gland. 

In  a  general  way,  it  may  be  said  that  man  is  subject  to 


FIG.  112.— Diagram 
showing  the  position  of 
the  thyroid  gland.  TC, 
thyroid  cartilage ;  TG, 
thyroid  gland;  T , 
trachea.  The  parathy- 
roids are  indicated  in 
black. 


THE  INTERNAL  SECRETIONS 


297 


either  an  increased  or  a  decreased  output  of  anyone  of  the 
endocrine  products.  Either  condition  usually  gives  rise  to 
a  complex  of  perfectly  definite  symptoms.  In  the  case  of 
the  thyroid,  it  has  been  established  by  clinical  studies  that  a 
lowered  production  of  its  active  principle  leads  to  cretinism 


A  B 

FIG.    113. — Cretin    before    (A)    and    after    (B)    treatment  with  sheep's 
thyroid.     (Nicholosn.) 

and  myxedema.  The  former  is  a  deficiency  disease  of  infancy, 
and  the  latter  a  disease  of  adult  life.  Cretinism  or  infantilism 
means  that  the  infant  is  dwarfed  in  its  stature,  owing  to  a 
retardation  in  the  growth  of  its  bones  and  soft  parts.  The 
face  presents  a  swollen  appearance;  the  features  are  im- 
perfectly outlined;  the  hair  is  scanty  and  the  skin  thick  and 


298  NUTRITION 

dry.  Mentally,  the  cretins  are  far  behind  children  of  the 
same  age.  This  condition  may  readily  be  remedied  in  the 
course  of  a  few  weeks  by  the  careful  administration  of  extract 
of  thyroid.  Very  similar  symptoms  may  develop  in  adults, 
giving  rise  to  the  condition  of  myxedema.  These  symptoms 
also  yield  to  the  feeding  of  thyroid  substance. 

Too  copious  a  production  of  thyroid  secretion  gives  rise 
to  an  extreme  irritability  of  the  nervous  system,  trembling 
of  the  muscles,  and  psychic  exultation.  When  these  symp- 
toms finally  become  associated  with  attacks  of.  tachycardia  or 
palpitation  of  the  heart  and  bulging  of  the  eyes  or  exophthal- 
mos,  a  distinct  clinical  picture  is  produced  which  is  commonly 
termed  Basedow's  disease  or  Grave's  disease.  While  this 
functional  disturbance  usually  leads  to  the  death  of  the 
patient,  if  allowed  to  develop  fully,  it  may  be  remedied  soon 
after  its  onset  by  the  partial  excision  of  the  thyroids.  About 
one-third  of  the  total  substance  of  these  organs  must  always 
be  left  behind,  because  the  removal  of  too  large  a  portion  is 
invariably  followed  by  death. 

It  is  to  be  noted,  however,  that  the  thyroids  need  not 
be  markedly  enlarged,  although  yielding  excessive  amounts 
of  secretion.  Contrariwise,  it  is  a  well  known  fact  that  those 
persons  who  are  afflicted  with  goiter  or  enlarged  thyroids, 
need  not  present  symptoms  indicative  of  a  superfluous 
secretion  of  thyroid.  In  most  instances  the  development  of 
a  goiter  merely  suggests  an  unusual  growth  of  the  framework 
of  this  gland,  which  often  seriously  interferes  with  the  flow 
of  the  respiratory  air. 

These  clinical  studies,  as  well  as  animal  experimentation, 
have  shown  that  the  thyroid  furnishes  an  internal  agent 
which  plays  an  important  part  in  the  metabolism  of  the  tissues, 
chiefly  those  of  the  nervous  system.  The  product  of  the 
parathyroids  is  concerned  with  the  elimination  of  certain 
toxic  substances  formed  in  the  course  of  metabolism.  When 
these  glandular  structures  are  removed,  these  substances 
accumulate  in  the  system  and  finally  produce  the  clinical 
picture  of  tetany,  consisting  of  muscular  tremors  and  spasms 
as  well  as  of  a  loss  of  the  tonus  and  coordination  of  the 
skeletal  musculature. 


THE  INTERNAL  SECRETIONS 


299 


The  Adrenal  Glands. — These  small  pea-shaped  organs  are 
situated  one  above  each  kidney  (Fig.  114).  They  are 
supplied  by  several  minor  branches  of  the  aorta  and  pour 
their  venous  drainage  into  the  suprarenal  vein,  a  tributary 
of  the  inferior  vena  cava.  It  has  been  established  that  their 
total  removal  leads  to  the  death  of  the  animal  within  a  few 
days,  while  the  extirpation  of  only  one  organ  does  not. 
Likewise,  it  is  possible  to  remove  one  gland  and  to  transplant 


FIG.  114. — Diagram  to  illustrate  the  position  of  the  adrenal  glands 
(rabbit).  K,  kidneys;  V,  ureters;  R  V,  renal  veins;  RA,  renal  arteries; 
JC,  inferior  vena  cava;  A,  abdominal  aorta;  S,  adrenal  glands;  *SC7,  supra- 
renal veins.  In  man,  the  two  kidneys  lie  very  nearly  in  the  same  hori- 
zontal plane ;  in  fact,  the  right  organ  frequently  below  the  left. 

the  opposite  one  into  some  other  region  of  the  body  without 
producing  fatal  results.  These  facts  clearly  prove  that 
these  bodies  furnish  an  internal  product  which  is  absolutely 
essential  to  the  life  of  the  animal.  This  conclusion  finds 
additional  support  in  the  clinical  picture  presented  by  persons 
suffering  from  Addison's  disease.  It  has  been  established 
that  the  degeneration  or  destruction  of  these  bodies  in  man 
gives  rise  to  muscular  weakness  and  tremors,  convulsions, 
and  a  decided  bronzing  of  the  skin.  This  disease  usually 
proves  fatal  within  a  few  months. 


300  NUTRITION 

The  chief  function  of  the  adrenal  glands  consists  in  the 
production  of  an  agent  which  constricts  the  bloodvessels. 
It  is  known  as  adrenin.  This  drug-like  body  is  secreted  by 
the  medullary  portions  of  these  glands  and  is  then  passed 
into  the  suprarenal  veins,  whence  it  reaches  the  venous 
circulation.  On  being  eventually  diverted  into  the  arteries 
and  arterioles,  it  causes  the  smooth  muscle  fibers  of  these 
channels  to  contract.  It  need  scarcely  be  mentioned  that 
this  constriction  of  the  arterioles  retards  the  escape  of  the 
arterial  blood  into  the  capillaries  and  gives  rise  to  an  increase 
in  the  arterial  bloodpressure. 

In  accordance  with  this  characteristic  action  of  adrenin,  it 
is  believed  that  these  glands  constantly  secrete  minimal 
amounts  of  this  agent  which  thus  tend  to  retain  the  blood- 
vessels in  a  condition  of  semi-constriction  or  tonus.  A  more 
copious  outpouring,  however,  may  result  at  any  moment  in 
consequence  of  reflex  stimuli.  Thus,  it  has  been  stated  that 
.  its  output  is  considerably  increased  during  emotional  states, 
induced  by  fear  and  anger.  In  consequence  of  this  in- 
creased production  of  adrenin,  larger  amounts  of  sugar  are 
mobilized  which  find  their  way  into  the  urine,  thereby 
establishing  the  condition  of  emotional  glycosuria. 

The  active  principle  adrenin  is  usually  obtained  by  mac- 
erating these  glands  and  extracting  them  in  normal  saline 
solution.  This  extract  is  then  injected  into  the  bloodstream. 
The  commercial  preparation  of  it,  which  is  known  as  adrena- 
lin, is  employed  to  stop  bleeding,  because  when  applied  to 
an  injured  surface,  it  constricts  the  opened  vessels,  thereby 
diminishing  the  flow  of  the  blood.  While  this  action  is 
only  temporary,  it  usually  lasts  a  sufficient  length  of  time 
to  allow  coagulation  to  set  in.  The  bleeding  points  are  then 
blocked  in  a  more  effective  and  permanent  manner.  It 
might  also  be  mentioned  that  adrenalin  tends  to  overcome 
muscular  fatigue,  because  it  intensifies  the  entire  circulation. 
Its  medicinal  use  for  this  particular  purpose,  however,  is 
contra-indicated  physiologically. 

The  Liver  and  Pancreas. — Repeated  mention  has  been 
made  of  the  fact  that  the  sugar  of  the  portal  blood  is  stored 
in  the  cells  of  the  liver  in  the  form  of  glycogen,  and  that  this 


THE  INTERNAL  SECRETIONS  301 

animal-starch  may  again  be  converted  into  circulating  sugar 
by  these  cells.  It  is  assumed,  therefore,  that  the  liver 
produces  an  inherent  agent  to  which  these  chemical  changes 
are  assigned.  Likewise,  it  has  been  stated  that  the  cells 
of  the  islands  of  Langerhans  of  the  pancreas  furnish  an  inter- 
nal secretion  which  changes  the  circulating  sugar  into  a 
form  adapted  to  the  chemical  powers  of  the  tissue  cells. 
Sugar  which  has  not  been  acted  upon  in  this  way,  cannot  be 
oxidized  so  readily  and  must,  therefore,  remain  behind  in  the 
blood  and  eventually  be  excreted  by  the  kidneys.  This 
metabolic  disturbance  is  one  of  the  fundamental  causes  of 
diabetes  mellitus. 

The  Pituitary  Gland. — This  structure  lies  at  the  base  of 
the  brain,  occupying  here  a  recess  in  the  sphenoid  bone.  It 
appears  as  a  reddish-gray  mass  of  about  the  size  of  a  pea, 
and  is  connected  with  the  main  mass  of  the  cerebrum  by  a 
peduncle.  It  consists  of  two  principal  portions  which  are 
designated  respectively  as  its  anterior  and  posterior  lobe. 
The  former  appears  to  be  connected  in  some  way  with  the 
metabolism  of  the  bones,  this  conclusion  being  based  upon 
the  fact  that  the  pituitary  is  usually  enlarged  in  all  cases  of 
giant  growth  or  acromegaly.  This  form  of  gigantism  affects 
the  bones  of  the  face  as  well  as  those  of  the  fingers  and  toes, 
rendering  them  club-shaped  and  frequently  causing  a  general 
contortion  of  the  part.  Accordingly,  it  is  thought  entirely 
probable  that  the  unusual  length  of  the  long  bones  of  giants 
finds  its  cause  in  an  excessive  production  of  this  particular 
hormone.  Contrariwise,  a  diminished  secretion  of  this 
active  principle  must  retard  the  growth  of  the  bones,  and 
give  rise  to  the  condition  of  dwarfism. 

The  posterior  lobe  of  the  pituitary  consists  of  a  tissue 
which  bears  a  close  resemblance  to  the  framework  of  nervous 
tissue.  It  does  not  seem  to  possess  a  specific  function. 
That  portion  of  the  posterior  lobe,  however,  which  lies  in 
contact  with  the  anterior  lobe,  consists  of  a  row  of  lining 
cells  possessing  a  true  secretory  character.  When  injected 
into  the  bloodstream,  the  extract  of  this  particular  segment 
of  the  pituitary  gland  evokes  a  constriction  of  the  blood- 
vessels and  rise  in  the  arterial  pressure  very  similar  to  that 


302  NUTRITION 

noted  after  the  administration  of  adrenalin.  Repeated 
experimentation  has  shown  that  the  hormone  secreted  by 
these  cells  possesses  a  stimulating  action  upon  the  smooth 
muscle  tissue  throughout  the  body  and  also  excites  secre- 
tion; for  example,  that  of  milk. 

The  Thymus. — This  organ  occupies  a  position  at  the  base 
of  the  heart  and  in  front  of  the  great  vessels.  The  size  of 
this  organ  differs  considerably  in  accordance  with  the  age 
of  the  person.  In  infants,  for  instance,  its  average  weight  is 
12  grams,  at  puberty  35  grams,  and  at  sixty  years  less  than 
15  grams.  Relatively  speaking,  therefore,  it  is  much  larger 
early  in  life  than  after  puberty.  Its  structure  is  similar  to 
that  of  lymphoid  tissue  and  shows  a  division  into  a  cortical 
and  medullary  portion. 

Regarding  the  function  of  this  organ  very  little  is  known. 
It  is  obviously  metabolic  in  its  function,  attaining  its 
greatest  importance  at  ,the  time  of  puberty.,  Inasmuch 
as  it  atrophies  during  adult  life,  it  is  supposed  to  be 
connected  with  the  development  of  the  sexual  organs  and 
characteristics. 

The  Spleen. — While  it  has  been  proven  that  the  spleen 
does  not  furnish  an  internal  secretion,  its  function  is  as  yet 
so  much  in  doubt  that  great  difficulty  is  experienced  in  classi- 
fying it  in  its  proper  relation  to  other  structures.  It  is 
situated  somewhat  below  and  toward  the  left  side  of  the 
stomach  and  possesses  a  flat,  elongated  outline.  Its  blood- 
supply  is  furnished  by  the  splenic  artery,  a  branch  of  the 
co3liac  axis  of  the  abdominal  aorta,  while  its  venous  drainage 
is  conveyed  into  the  portal  vein.  When  its  capsular  sheath 
is  removed,  it  will  be  found  that  partitions  of  connective 
tissue  or  trabeculse  enter  its  interior  and  subdivide  the 
entire  organ  into  a  number  of  spaces  which  are  filled  with 
spongy  material,  called  the  spleen-pulp.  The  meshes  of  the 
pulp  are  occupied  by  red  and  white  blood  corpuscles  as  well 
as  by  large  giant  cells,  possessing  amosboid  qualities.  The 
organ  as  a  whole  presents  the  characteristics  of  lymphoid 
tissue,  such  as  forms  the  principal  mass  of  the  lymphatic  glands. 

Inasmuch  as  this  organ  may  be  removed  without  serious 
consequences,  it  may  be  concluded  that  it  does  not  furnish  a 


THE  INTERNAL  SECRETIONS  303 

specific  internal  secretion.  Furthermore,  the  fact  that  it  is 
not  indispensible  to  the  life  of  the  animal,  leads  us  to  suspect 
that  its  function  may  be  transferred  at  any  time  to  other 
organs.  Its  lymphoid  structure  brings  to  our  minds  first 
of  all  the  possibility  that  it  forms  white  blood  corpuscles. 
But,  since  lymphoid  tissues  in  general  are  engaged  in  this 
process,  the  removal  of  this  organ  merely  causes  the  other 
tissues  of  like  character  to  compensate  for  this  loss. 

The  spleen  is  also  one  of  the  organs  in  which  the  red  blood 
corpuscles  are  destroyed.  This  conclusion  is  based  upon  the 
fact  that  the  splenic  pulp  is  loaded  with  fragmented  red 
cells.  It  should  be  remembered,  however,  that  this  organ 
is  not  the  only  place  in  which  these  corpuscles  are  destroyed. 
A  far  more  important  destructive  power  is  possessed  by  the 
liver.  Thus,  it  is  commonly  believed  that  the  spleen  merely 
instigates  the  disintegration  of  the  senile  red  corpuscles, 
permitting  the  liver  to  reduce  them  further  into  their  ele- 
mentary constituents.  It  has  been  noted  in  a  preceding 
chapter  that  the  pigments  of  the  bile  as  well  as  the  iron  are 
derived  from  these  cells. 

Owing  to  the  spongy  character  of  the  substance  of  the 
spleen,  allowing  this  organ  to  become  highly  distended,  it 
has  also  been  thought  that  it  serves  as  a  sort  of  reservoir  for 
the  portal  blood.  Thus,  it  is  believed  by  some  physiologists 
that  it  accommodates  varying  amounts  of  blood,  thereby 
giving  rise  to  a  more  equal  distribution  of  the  blood  when 
required  for  purposes  of  digestion. 


PART  V 
THE  NERVOUS  SYSTEM 

CHAPTER  XXX 

THE   FUNCTIONAL   DEVELOPMENT    OF   THE 
NERVOUS  SYSTEM 

The  General  Arrangement  of  the  Nervous  System. — The 
many  millions  of  neurones  composing  the  nervous  system, 
are  moulded  into  complex  masses  of  tissue,  the  minute 
arrangement  of  which  can  only  be  thoroughly  understood 
after  considerable  study.  In  general,  however,  it  may  be 
stated  that  the  nervous  system  consists  of  a  central  and  a 
peripheral  part.  The  central  one  embraces  the  cerebrum, 
cerebellum,  basal  ganglia,  medulla  oblongata,  and  spinal 
cord,  while  the  peripheral  one  is  composed  essentially  of  an 
intricate  network  of  nerves  and  a  large  number  of  ganglia. 
Twelve  pairs  of  these  nerves  are  given  off  from  the  cerebrum 
and  the  region  of  the  pons  and  medulla,  while  thirty-one 
pairs  arise  from  the  spinal  cord  itself. 

The  peripheral  extent  of  the  nervous  system  also  embraces 
numerous  nerve  fibers  and  ganglia  which  control  the  func- 
tions of  the  viscera  and  are  not  under  the  control  of  the  will. 
They  form  what  is  known  as  the  autonomic  nervous  system. 
Hence,  it  will  be  seen  that  the  nervous  system  may  also  be 
divided  into  a  cerebrospinal  part  and  an  autonomic  part. 
The  former  embraces  the  aforesaid  central  masses  of  nerve 
tissue  as  well  as  the  twelve  pairs  of  cranial  and  thirty-one 
pairs  of  spinal  nerves.  Correspondingly,  the  latter  is  made 
up  of :  (a)  a  number  of  ganglia  and  nerves  which  are  situated 
along  the  thoracic  division  of  the  spinal  cord,  and  (6)  a 
number  of  ganglia  and  nerves  located  along  the  cranial 

304 


FUNCTIONAL  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM    305 


nerves  and  sacral  segment  of  the  spinal  cord.  For  this 
reason,  it  is  usually  stated  that  the  autonomic  system  belongs 
to  the  peripheral  nervous  system  and  is  composed  of  a 
sympathetic  and  a  parasympathetic  division.  The  following 
outlines  may  be  of  service  in  understanding  this  classification : 


Nervous  system 


Central 


Peripheral 


Cerebrum 
Cerebellum 
Basal  ganglia 
Medulla 
Spinal  cord 
Cranial  nerves 
Spinal  nerves 


Nervous  system 


Cerebro-spinal 
(medullated) 


Autonomic 
(non-  medullated ) 


Cerebrum 
Cerebellum 
Basal  ganglia 
Medulla 
Spinal  cord 
Cranial  nerves 
Spinal  nerves 

Parasympathetic  ramifications 
and  ganglia  from  the  cranial 
and  sacral  spinal  nerves. 
Sympathetic  ramifications  and 
ganglia  from  the  dorsal  spinal 
nerves. 


The  General  Function  of  the  Nervous  System. — The 
many  millions  of  cells  composing  the  body  of  the  higher 
animal,  work  in  groups,  each  fulfilling  a  particular  purpose 
in  order  to  achieve  a  functional  whole.  Hence,  the  body 
may  be  likened  to  a  large  manufacturing  establishment,  in 
which  each  department  turns  out  its  own  particular  contri- 
vance to  be  later  on  joined  with  others  into  a  complex  piece 
of  machinery.  In  order  to  effect  a  harmonious  and  purpose- 
ful working  of  these  different  departments,  a  close  correlation 
must  be  established  between  them  through  foremen  who  in 
turn  are  directed  by  a  manager. 

Many  parts  of  our  body,  and  principally  those  subserving 
the  vegetative  processes,  are  able  to  act  independently  of  one 
another,  but  the  best  results  can  only  be  obtained  if  the 

20 


306  THE    SENSE-ORGANS 

functions  of  the  different  organs  are  correlated  to  yield  a 
common  general  product.  For  this  reason,  nerve  centers  and 
nerve  paths  have  been  established  through  which  the  differ- 
ent constituents  of  the  body  are  enabled  to  exchange  im- 
pulses signifying  their  needs  to  the  governing  body  of  the 
entire  mechanism.  Likewise,  impulses  may  in  this  way 
be  relayed  to  them  informing  them  about  the  functional 
requirements  of  other  parts.  The  neurones  accomplishing 
this  coordination,  are  always  arranged  in  such  a  way  that 
their  conductile  elements  form  definite  paths  or  nerves, 
while  their  generating  parts  or  cell-bodies  are  combined 
into  nuclei  and  centers.  Thus,  we  may  justly  draw  the 
general  conclusion  that  the  central  nervous  system  is  the 
seat  of  many  centers  controlling  the  actions  of  the  peripheral 
parts,  while  the  nerves  merely  serve  the  purpose  of  bringing 
the  latter  into  functional  relation  with  the  former. 

The  different  parts  of  our  body  must  accomplish  the 
right  things  at  the  proper  time.  When  a  muscle  contracts 
it  is  made  to  do^  so  by  a  nerve  impulse  conveyed  to  it  from 
its  center.  Furthermore,  its  contraction  must  be  coordinated 
with  those  of  other  muscles,  otherwise  a  purposeful  action 
cannot  be  obtained.  The  same  statement  may  justly  be 
made  regarding  other  organs.  Thus,  we  have  seen  that  the 
flow  of  the  digestive  secretions  is  accurately  timed  so  as  to 
permit  them  to  act  upon  the  foodstuffs  in  proper  order. 
Likewise,  the  calibre  of  the  bloodvessels  is  invariably  ad- 
justed in  a  way  to  correspond  to  the  energy  of  the  heart, 
otherwise  the  normal  height  of  the  bloodpressure  cannot 
easily  be  retained.  Quite  similarly,  the  bloodpressure  is 
altered  repeatedly  so  as  to  agree  with  the  varying  states  of 
activity  of  the  different  organs. 

The  Simple  Nervous  System. — Attention  has  already  been 
called  to  the  fact  that  the  structural  unit  of  the  nervous 
system  is  the  neurone,  and  that  the  most  elementary  nervous 
action  or  reflex  requires  the  presence  of  at  least  two  neurones. 
One  of  these  conducts  from  the  periphery  to  the  center,  and 
the  other  from  the  center  to  the  periphery.  The  former 
is  known  as  the  afferent  or  sensory  neurone  and  the  latter, 
as  the  efferent  or  motor  neurone.  Both  are  functionally 


FUNCTIONAL  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM    307 

related  to  one  another  by  the  synapse,  and  both  together 
constitute  what  is  known  as  a  reflex  circuit. 

These  reflex  circuits  are  first  noted  in  the  coelenterates. 
Below  this  species  nervous  elements  are  not  discernible, 
although  even  these  organisms  show  various  motor  reactions 
in  consequence  of  stimuli.  These  reactions,  however,  cannot 
be  called  reflexes,  because  they  are  accomplished  solely  by 
conduction  through  ordinary  protoplasm.  The  name  com- 
monly applied  to  them  is  reflex-like  reactions.  Thus,  it  will 
be  seen  that  the  animals  may  be  divided  into  two  classes: 
namely,  those  possessing  and  those  not  possessing  nervous 
tissue.  The  former  exhibit  simple  reflexes  as  well  as  voli- 
tional reactions  and  the  latter,  solely  reflex-like  reactions. 

We  are  now  chiefly  concerned  with  those  animals  whose 
functions  are  correlated  by  nervous  tissue.  Even  a  very 
casual  study  will  show  that  these  animals  may  in  turn  be 
divided  into  two  groups:  namely,  those  exhibiting  only 
simple  reflex  actions,  and  those  presenting  in  addition  com- 
plex volitional  responses.  Thus,  if  we  contrast  an  earthworm 
with  a  mammal,  it  will  be  evident  that  the  former  is  a  simple 
reflex  animal  and  absolutely  devoid  of  psychic  activities, 
whereas  the  behavior  of  the  latter  is  dominated  by  volition. 
This  statement,  however,  is  not  meant  to  imply  that  the 
mammal  does  not  present  simple  reflexes,  but  solely  that  its 
reflex  life  is  amplified  by  associations  and  their  consequent 
motor  reactions.  In  other  words,  the  simple  reflex  life  of 
the  lower  forms  is  dominated  in  the  higher  animals  by  certain 
psychic  products. 

The  Segmental  Animal. — Reflex  action  may  be  studied  best 
in  such  animals  as  the  vermes  and  crustacese.  Especially 
the  former  display  a  typical  segmental  arrangement,  i.e., 
their  bodies  are  composed  of  a  number  of  segments  which  are 
in  possession  of  separate  organs.  Thus,  each  portion  of 
their  body  may  lead  a  practically  independent  existence, 
although  all  are  under  the  control  of  a  head-ganglion  or 
"brain"  which  causes  their  separate  actions  to  be  correlated 
for  the  attainment  of  a  common  purpose. 

Each  segment  contains  a  number  of  reflex  circuits,  the 
synapses  of  which  are  situated  centrally.  In  this  way,  a 


308 


THE    SENSE-ORGANS 


FIG.  115.— 
Diagrammatic 
representatio  n 
of  the  nervous 
system  of  the 
crayfish.  A, 
supraes  o  p  h  a- 
geal  ganglion ; 
B  commis- 
sure ;  C,  sub- 
esophageal 
ganglion ;  Z), 
first  abdomi- 
nal ganglion ; 
O,  optic  nerve; 
P,  antennary 
nerve;  S, 
stomatogastric 
nerve. 


main  ganglion  is  formed  which  serves  as  the  con- 
trolling agent  of  each  segment.  These  ganglia 
are  connected  with  one  another  by  longi- 
tudinal neurones  which  thus  establish  com- 
munication between  them  as  well  as  with  the 
head-ganglion  or  "  brain."  These  intergangli- 
onic  fibers  form  the  beginning  of  the  spinal 
cord  of  the  higher  animals.  The  chief  purpose 
of  this  structure  is  that  of  a  highway  connecting 
the  brain  with  peripheral  parts. 

The  essential  points  of  this  discussion  may 
readily  be  reviewed  with  the  aid  of  Fig.  115 
which  represents  the  nervous  system  of  the 
crayfish,  schematically  outlined.  It  consists 
of  thirteen  ganglia,  six  of  which  belong  to  the 
abdomen,  six  to  the  thorax,  and  one  to  the 
head.  Each  ganglion  controls  the  correspond- 
ing portion  of  the  body,  and  communicates 
with  the  neighboring  ganglia  as  well  as  with 
the  head-ganglion.  The  latter  receives  diverse 
sensory  impressions  from  the  receptors  for 
sight,  hearing,  and  touch,  and  is  thereby  en- 
abled to  exert  a  much  greater  influence  upon 
the  animal  as  a  whole  than  the  other  ganglia. 

The  Development  of  the  Association  Realms. 
Volition. — While  this  simple  reflex  system  is 
also  present  in  the  higher  animals,  it  has  been 
considerably  amplified  by  the  development  of 
an  additional  number  of  neurones  which  sub- 
serve psychic  activities.  For  the  present,  these 
higher  activities  may  be  designated  in  brief 
as  associations.  In  the  earthworm  or  crayfish 
an  impact  upon  the  integument  gives  rise  to  a 
musculo-motor  reaction  without  involving 
volition  or  similar  psychic  processes.  The 
same  direct  course  is  followed  by  those 
afferent  stimuli  which  ascend  from  the  retina 
of  the  eye  or  the  organ  of  hearing. 

In  the  higher   animals,  these  simple  reflex 


FUNCTIONAL  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM    309 

processes  are  subordinated  to  the  activities  of  the  association 
centers,  situated  in  the  more  recently  developed  cerebrum. 
The  outer  realm  or  cortex  of  this  structure  embraces  a  large 
number  of  ganglion  cells  which  are  set  aside  for  the  purpose 
of  associating  many  sensory  impulses  before  they  are  actually 
allowed  to  pass  on  to  the  motor  end-organs.  In  this  way, 
it  is  made  possible  to  activate  a  certain  effector  not  only  in  a 
reflex  way  but  also  volitionally.  When  the  cornea  of  the 
eye  is  touched,  the  eyelids  are  closed  in  order  to  protect  the 
eyeball  against  injury.  This  act  is  non- volitional  in  its 
nature  and  is,  therefore,  a  simple  reflex.  But,  it  is  also 
possible  to  close  the  eyelids  volitionally  in  consequence 
of  certain  associations  formed  in  the  corresponding  center  of 
the  cerebrum.  Likewise,  an  intense  sound  or  other  sensory 
impression  may  lead  either  to  an  almost  instantaneous 
protective  muscular  reflex,  or  may  first  be  relayed  into  the 
corresponding  association  center  in  the  cerebrum  to  undergo 
certain  modifications.  In  the  latter  instance,  it  may  be 
inhibited  or  may  be  permitted  to  effect  a  motor  response  of  a 
more  complex  and  purposeful  character.  In  other  words, 
it  is  then  closely  dominated  by  volition. 

Naturally,  the  development  of  the  cerebral  association 
areas  places  the  animal  upon  a  level  considerably  above  that 
occupied  by  the  simple  reflex  animals.  It  is  to  be  noted, 
however,  that  this  development  takes  place  gradually,  its 
beginning  being  noted  in  the  reptilia  and  amphibia  and  its 
highest  stage  in  man.  Thus,  we  find  that  the  simple  reflex 
system  of  the  frog  and  turtle  has  been  amplified  by  the 
formation  of  a  relatively  small  cerebrum,  the  principal 
part  of  which  is  occupied  by  the  association  center  for  smell, 
the  olfactory  lobes.  Since  the  existence  of  these  animals 
is  closely  dependent  upon  their  purposeful  behavior  towards 
these  sensations,  it  cannot  surprise  us  to  find  that  they  are 
placed  in  a  particularly  favorable  position  to  analyze  them. 

Another  important  complex  of  nervous  tissue  is  the  hind- 
brain  or  cerebellum.  It  may  be  stated  at  this  time  that  this 
organ  serves  to  coordinate  the  different  muscular  movements, 
so  that  they  may  be  executed  with  precision  and  in  harmony 
with  one  another.  Even  a  casual  observation  will  show  that 


310  THE    SENSE-ORGANS 

the  amphibia  and  reptilia  are  not  greatly  in  need  of  associa- 
tions of  this  kind,  because  their  movements  in  space  are 
static  in  character.  They  move  sluggishly  along  straight 
lines.  Thus,  it  is  found  that  the  cerebellum  of  the  frog  and 
turtle  is  rudimentary  in  size  and  structure.  Contrariwise, 
the  cerebellum  of  the  birds  presents  a  high  stage  of  develop- 
ment, because  these  animals  must  be  able  to  coordinate  their 
muscular  movements  very  precisely  in  order  to  retain  their 
equilibrium  and  to  be  able  to  orient  themselves  in  space. 


CHAPTER  XXXI 
THE    SPINAL    REFLEX    ANIMAL 

The  Development  of  the  Spinal  Cord. — It  has  previously 
been  emphasized  that  the  ganglia  of  the  different  portions  of 
the  segmental  animals  are  connected  with  one  another  by 
longitudinal  axones  which  thus  form  a  nerve  cord  traversing 
the  body  in  a  direction  from  before  backward.  A  similar 
arrangement  is  present  in  the  mammals.  These  animals  are 
in  possession  of  a  number  of  simple  reflex  centers  which  are 
arranged  in  series  and  are  connected  with  one  another  by 
longitudinal  bundles  of  nerve  fibers.  Accordingly,  there  are 
deposited  along  the  dorsal  aspect  of  these  animals  numerous 
ganglion  cells  which  are  united  not  only  with  the  peripheral 
motor  and  sensory  organs  but  also  with  one  another. 
Admittedly,  therefore,  the  basis  of  the  rudimentary  spinal 
cord  is  formed  by  neurones  subserving  simple  reflex  activities. 

The  subsequent  development  of  the  forebrain  and  hind- 
brain  necessitates  the  addition  to  this  reflex  system  of  numer- 
ous neurones  which  connect  the  spinal  ganglion  cells  with 
those  constituting  the  cerebrum  and  cerebellum.  Thus, 
there  is  built  up  upon  this  local  reflex  mechanism  of  the  cord 
a  second  one  which  bears  the  characteristics  of  a  long- 
conduction  system.  The  latter  establishes  communication 
not  only  between  the  brain  and  the  different  reflex  centers 
of  the  spinal  cord,  but  also  between  the  brain  and  the  distant 
motor  and  sensory  organs.  The  neurones  effecting  this  long 
distance  conduction,  form  the  so-called  projection  system. 
Hence,  the  spinal  cord  of  the  higher  animals  differs  from  that 
of  the  lower  forms  chiefly  in  that  it  contains  numerous  affer- 
ent and  efferent  projection  axones  as  well  as  many  ganglion 
cells  establishing  relay  stations  for  these  fibers. 

It  has  been  mentioned  in  one  of  the  preceding  chapters 
that  the  distance  between  the  cerebrum  and  the  motor  end- 

311 


312  THE    SENSE-ORGANS 

organs,  such  as  the  muscles  of  the  foot,  is  usually  covered 
by  two  efferent  neurones  arranged  in  series,  while  the  cor- 
responding sensory  path  is  generally  formed  by  three  con- 
secutive afferent  neurones.  It  is  also  to  be  noted  that  the 
connections  between  the  sensory  and  motor  end-organs  situ- 
ated in  the  region  of  the  head,  are  established  in  a  direct 
way  by  twelve  pairs  of  cranial  nerves.  The  last  six  of  these 
enter  the  cerebro-spinal  tract  in  the  region  of  the  medulla 
oblongata,  whereas  the  first  six,  such  as  the  nerves  of  smell 
and  sight,  enter  the  brain  above  this  structure. 

The  Spinal  Cord. — The  spinal  cord  of  man  appears  as  a 
cylindrical  structure,  measuring  from  40  to  45  cm.  in  length, 


RC 


FIG.  116. — Transverse  section  through  the  region  of  the  fourth  cervical 
vertebrae.  V,  body  of  vertebra;  B,  vertebral  blood-vessels;  N,  spinal 
nerve;  RC,  ramus  communicans ;  S,  spinal  ganglion;  A,  subarachnoidal 
space  investing  spinal  cord. 

and  12  mm.  in  diameter.  It  is  contained  in  the  vertebral 
canal,  which  is  formed  by  the  bodies  and  laminae  of  the  suc- 
cessive vertebra.  It  is  enveloped  by  membranes,  fatty 
tissue,  and  lymph.  Next  to  the  bony  wall  of  this  canal  lies 
the  dura  mater,  a  tough  fibrous  membrane,  serving  as  the 
periosteal  lining  of  these  bones.  Then  follows  the  arachnoid, 
and  lastly,  the  pia  mater.  The  arachnoid  is  a  delicate  mem- 
brane which  lies  in  rather  close  contact  with  the  dura,  but  is 
in  many  places  widely  separated  from  the  pia.  These  sub- 


THE    SPINAL    REFLEX    ANIMAL  313 

dural  and  subarachnoid  spaces  are  filled  with  a  lymph-like 
fluid  which  possesses  the  general  character  of  cerebro-spinal 
liquid. 

Beginning  at  the  level  of  the  atlas,  the  spinal  cord  gives 
off  thirty-one  pairs  of  nerves,  or,  in  general,  one  pair  for  each 
vertebra.  These  nerves  leave  the  vertebral  canal  through 

Dorsal  median  septum 

Septum 
Dorsal  lateral  groove 

Dorsal  nerve  root 

Substantia  gelatinosa 

Root-fibers  entering 
gray  matter 

Processus  reticularis 
Central  canal 


Nucleus  from  which 
motor  fibers  for  mus- 
cles of  upper  limb  arise 

Ventral  white  commis- 
sure 


Ventral  nerve  root 

Ventral  median  fissure 


FIG.  117. — Cross-section  through  the  human  spinal  cord  at  the  level 
of  the  fifth  cervical  nerve,  stained  by  the  method  of  Weigert-Pal,  which 
colors  the  white  matter  dark  and  leaves  the  gray  matter  uncolored. 
(From  Cunningham's  Anatomy.} 

apertures  between  the  vertebrae,  which  are  called  inter- 
vertebral  foramina.  The  adult  spinal  cord,  however,  does 
not  occupy  the  entire  length  of  the  vertebral  canal,  but  termi- 
nates opposite  the  second  or  third  lumbar  vertebra.  Below 
this  point  are  found  a  number  of  nerve  fibers  which  tarry  in 
the  wake  of  the  cord  before  they  actually  leave  this  canal. 
This  bundle  of  fibers  forms  the  filwn  terminate. 

If  a  cross-section  is  made  of  the  spinal  cord,  it  will  be  found 
to  embody  a  darker  central  mass  of  gray  matter  and  a  lighter 


314  THE    SENSE-ORGANS 

marginal  zone  of  white  matter.  The  former  presents  itself 
as  two  crescent-shaped  masses  which  are  connected  with  one 
another  by  a  narrow  bridge  or  commissure.  For  this  reason, 
it  exhibits  an  outline  similar  to  that  of  the  letter  H.  The 
white  matter  surrounds  this  central  core  of  gray  substance 
on  all  sides  in  the  form  of  a  relatively  narrow  capsule. 

Before  proceeding  further  brief  reference  should  be  made 
to  the  fact  that  this  peculiar  appearance  of  the  cross-section 
of  the  spinal  cord  is  occasioned  by  differences  in  the  arrange- 
ment of  its  neurones.  Gray  matter  consists  essentially  of  the 
cell-bodies  of  the  neurones  and  their  central  processes,  while 
white  matter  embraces  chiefly  axones  enveloped  by  their 
medullary  sheaths.  Accordingly,  it  will  be  seen  that  the 
outer  zone  of  the  spinal  cord  is  made  up  almost  exclusively  of 
nerve  fibers  conducting  impulses  in  a  longitudinal  direction 
through  the  cord,  while  the  cells  of  the  central  gray  matter 
serve  as  relay  stations  for  these  impulses,  enabling  them  to 
leave  this  structure  at  any  level  to  reach  the  corresponding 
segment  of  the  body. 

The  Spinal  Cord  as  an  Organ  of  Conduction. — The  cross- 
section  of  the  spinal  cord  presents  an  anterior  and  a  posterior 
fissure,  which  divide  this  entire  area  incompletely  into  a 
right  and  a  left  half.  Each  half  presents  three  principal 
columns  of  white  matter :  namely,  an  anterior,  a  lateral,  and 
a  posterior.  This  division  is  made  more  evident  by  the 
fact  that  each  spinal  nerve  arises  by  two  roots,  an  anterior 
and  a  posterior.  The  fibers  of  the  former  pierce  the  white 
matter  between  the  anterior  and  lateral  columns,  and  the 
latter,  between  the  posterior  and  lateral  columns. 

This  anatomical  arrangement  coincides  very  closely  with 
the  functional  character  of  the  nerve  fibers  composing  these 
columns  of  white  matter.  Thus,  it  may  briefly  be  stated  at 
this  time  that  the  spinal  cord  possesses  two  chief  functions: 
namely,  that  of  conduction,  and  that  of  a  center  for  reflex 
action.  If  we  confine  ourselves  for  the  present  to  its  func- 
tion as  a  highway  for  nerve  impulses,  it  should  be  emphasized 
that  its  white  matter  is  made  up  of  two  sets  of  fibers,  one  of 
which  connects  its  successive  segments  with  one  another  and 
one  which  establishes  communication  between  the  brain  and 


THE    SPINAL    REFLEX    ANIMAL 


315 


distal  parts.     Both  sets  embrace  afferent  as  well  as  efferent 
axones. 

If  we  regard  this  subject-matter  in  a  very  general  way, 
it  may  be  stated  that  those  fibers  which  are  concerned  with 
reflex  action,  occupy  a  position  next  to  the  gray  matter, 
while  those  belonging  to  the  projection-system  are  arranged 
as  definite  outer  bundles.  Secondly,  those  fibers  which 


• — m 


FIG.  118. — Conduction  in  the  spinal  cord.  AF,  anterior  fissure; 
PF,  posterior  fissure;  AC,  anterior  column;  LC,  lateral  column; 
PC,  posterior  column;  1,  direct  pyramidal  tract;  2,  anterior  ground 
bundle;  3,  lateral  ground  bundle;  4,  Gower's  tract;  5,  Flechsig's  tract; 
6,  crossed  pyramidal  tract;  7,  column  of  Burdoch;  8,  column  of  Goll; 
AR,  anterior  root;  PR,  posterior  root;  SG,  spinal  ganglion ;  SN,  spinal  nerve. 

conduct  in  an  afferent  direction,  occupy  chiefly  the  posterior 
column  and  to  some  extent  also  the  lateral  column.  Con- 
trariwise, those  fibers  which  convey  impulses  in  an  efferent 
direction,  descend  through  the  anterior  column  and  in  a 
measure  also  through  the  lateral  column.  It  is  to  be  noted, 
therefore,  that  the  white  matter  of  the  spinal  cord  is  sub- 
divided into  distinct  bundles  of  nerve  fibers  subserving  con- 
duction in  special  directions.  These  tracts  are  either  afferent 
(ascending)  or  efferent  (descending)  in  their  character. 


316  THE    SENSE-ORGANS 

This  division  of  function  is  in  entire  agreement  with  the 
localization  of  conduction  exhibited  by  the  roots  of  the  spinal 
cord.  The  fibers  leaving  the  cord  in  front  are  efferent, 
which  implies  that  the  cell-bodies  of  these  neurones  are  sit- 
uated in  the  anterior  segment  or  horn  of  the  gray  matter,  while 
their  axones  pursue  a  course  outward  through  the  anterior 
root.  Contrariwise,  the  fibers  forming  the  posterior  root, 
conduct  in  an  afferent  direction.  Their  cell-bodies  are 
situated  near  the  junction  of  the  anterior  and  posterior 
roots,  where  they  form  a  small  nodule  which  is  designated 
as  the  spinal  ganglion  (Fig.  118).  Distally  to  this  point, 
the  fibers  of  the  posterior  root  intermingle  with  those  of  the 
anterior  root,  forming  the  spinal  nerve  proper.  Centrally 
to  the  spinal  ganglion,  however,  the  afferent  fibers  pursue  a 
separate  course  and  finally  enter  the  posterior  realm  or  horn 
of  the  gray  matter.  Those  impulses  which  are  to  be  conve}red 
to  higher  levels  of  the  cord  and  the  cerebrum,  are  transferred 
to  the  ascending  fibers  of  the  posterior  tracts,  and  those  in- 
tended for  the  cerebellum,  to  the  corresponding  ascending 
lateral  tracts.  It  need  scarcely  be  emphasized  that  these 
afferent  fibers  are  derived  from  peripheral  sense-organs  or 
receptors,  while  the  efferent  fibers  eventually  terminate  in 
motor-organs  or  effectors. 

The  Spinal  Cord  as  an  Organ  of  Reflex  Action. — The  fact 
that  the  spinal  cord  of  the  higher  animals  aids  in  reflex  action, 
may  readily  be  proved  by  removing  the  brain  of  an  etherized 
frog,  and  by  subjecting  this  brainless  animal  later  on  to 
stimulations.  In  accordance  with  the  preceding  discussions, 
it  must  be  evident  that  the  destruction  of  the  cerebral 
association  centers  converts  this  animal  into  a  reflex  machine 
which  is  incapable  of  perceiving  pain  and  of  receiving  any 
sensory  impulse  in  "  consciousness."  The  ordinary  afferent 
and  efferent  impulses,  however,  are  not  destroyed,  excepting 
those  which  in  part  traverse  the  higher  segments  of  the 
nervous  system. 

If  the  foot  of  such  a  "reflex  frog"  is  stimulated  by  immers- 
ing it  in  very  dilute  acetic  acid  or  by  pinching  it  slightly  with 
the  forceps,  the  muscles  of  this  leg  contract.  The  foot  is  then 
removed  from  the  seat  of  the  stimulation.  This  reaction 


THE    SPINAL    REFLEX    ANIMAL  317 

follows  after  an  appreciable  interval,  which  becomes  the 
shorter  the  stronger  the  stimulus.  The  time  intervening 
between  the  moment  of  the  application  of  the  stimulus  and 
the  muscular  reaction,  is  called  the  reflex-time. 

It  is  also  to  be  noted  that  a  somewhat  greater  strength 
of  stimulus  eventually  evokes  movements  of  the  opposite 
hind-leg,  abdominal  parts,  and  fore-legs.  This  result 
leads  us  to  infer  that  a  strong  stimulus  involves  not  only 
the  reflex  circuits  of  the  leg  stimulated,  but  also  those  in 
adjoining  parts  of  the  body.  This  phenomenon  is  desig- 
nated as  spreading  of  impulses  or  spreading  of  reflexes. 
It  may  also  be  evoked  by  increasing  the  irritability  of  the 
nervous  system  as  a  whole.  A  drug  commonly  used  for  this 
purpose  is  strychnin,  which  is  believed  to  establish  a  more 
intimate  connection  between  the  sensory  terminals  and 
dendrites  of  the  motor  neurone.  Because  of  this  change  in 
the  synapses,  the  sensory  impulses  are  enabled  to  influence  a 
much  greater  number  of  motor  neurones.  Thus,  it  is  a  well 
known  fact  that  even  the  slightest  mechanical  stimulus 
applied  to  the  skin  of  a  frog  poisoned  with  strychnin,  will 
give  rise  to  very  extensive  and  prolonged  muscular  spasms. 

All  these  reflex  contractions  of  the  muscles  cease  immedi- 
ately after  the  spinal  cord  has  been  destroyed.  This  result 
prompts  us  to  conclude  that  this  structure  is  absolutely 
essential  for  these  reflex  responses,  because  it  contains 
the  synapses  of  the  afferent  and  efferent  neurones  concerned 
in  these  reactions.  For  this  reason,  it  is  commonly  stated 
that  the  cord  is  an  important  seat  of  reflex  activity.  It 
should  be  noted,  however,  that  it  is  not  the  only  part  of  the 
nervous  system  set  aside  for  this  function,  because  reflexes 
may  also  be  obtained  in  the  domain  of  the  cranial  nerves, 
as  well  as  in  that  of  the  sympathetic  nervous  system. 

In  the  amphibians  and  reptilians  it  is  also  evident  that 
these  spinal  reflex  centers  are  localized  in  particular  segments 
of  the  cord.  Thus,  it  may  readily  be  proved  by  destroying 
different  portions  of  the  spinal  cord  of  the  frog  that  the 
reflexes  evoked  from  the  hind-legs,  are  controlled  by  ganglion 
cells  which  are  situated  opposite  the  seventh  and  eighth 
vertebrae.  Quite  similarly,  it  may  be  shown  that  the  synap- 


318  THE    SENSE-ORGANS 

ses  of  the  neurones  concerned  with  the  reflex  responses  of  the 
fore-legs,  are  placed  opposite  the  third  and  fourth  vertebrae. 
In  locating  these  centers,  it  should  be  remembered  that  the 
spinal  column  of  the  frog  consists  of  only  nine  vertebrse. 
The  tenth  vertebra  is  modified  to  form  the  dorsal  wall  of  the 
very  extensive  pelvis.  The  point  of  union  between  these 
two  vertebrse  lies  at  the  prominence  upon  the  dorsal  aspect 
of  the  body  of  this  animal. 

While  it  may  be  granted  that  this  localization  of  reflex 
function  is  not  so  evident  in  the  higher  animals  as  in  the  lower, 
it  is  nevertheless  quite  obvious  that  the  spinal  cord  of  the 
former  embraces  a  number  of  simple  centers  which  are  con- 
cerned with  the  processes  of  micturition,  defecation,  and 
reproduction.  Besides,  this  structure  displays  a  distinct 
segmental  arrangement,  because  each  spinal  nerve  is  appor- 
tioned very  nearly  to  that  segment  of  the  body  which  lies 
opposite  its  origin.  The  lowest  nerves,  however,  bend  back 
considerably  in  order  to  reach  the  posterior  parts  of  the  body. 
This  arrangement  is  clearly  portrayed  by  the  sciatic  nerve, 
the  nucleus  of  which  lies  in  the  lumbar  segment  of  the  spinal 
cord,  while  its  fibers  pass  almost  directly  backward. 

Examples  of  Reflex  Action. — By  means  of  the  sciatic  nerve 
it  is  possible  to  evoke  a  reflex  which  is  no  doubt  familiar  to 
every  one.  It  is  termed  the  patellar  reflex,  and  is  elicited 
by  tapping  upon  the  patellar  ligament,  while  the  leg  is 
suspended  across  the  edge  of  a  chair  or  table.  The  impulses 
so  generated  in  this  locality,  are  conveyed  to  the  sciatic  I 
center,  and  thence  outward  to  the  quadriceps  f emoris  muscle.  ^ 
On  contracting,  this  muscle  extends  the  leg  upon  the  thigh. 
The  varying  intensity  of  this  reflex  permits  us  to  form  an  idea 
regarding  the  state  of  irritability  of  the  entire  nervous  system, 
and  to  locate  lesions  of  its  constituent  parts.  Injuries  to  the 
cord  frequently  abolish  this  reaction  entirely,  while  lesions 
of  the  higher  centers  increase  its  intensity. 

It  would  be  incorrect,  however,  to  gain  the  impression  that 
reflexes  invariably  consist  of  movements.  Thus,  we  have 
previously  noted  that  the  flow  of  saliva,  gastric  juice  and 
pancreatic  juice  is  the  direct  result  of  stimuli  brought  to  bear 
upon  the  secretory  elements  of  these  glands.  We  have  also 


THE    SPINAL    REFLEX    ANIMAL  319 

become  acquainted  with  the  reflex  character  of  the  acts  of 
sneezing  and  coughing,  the  closure  of  the  eyelids,  vomiting, 
the  dilatation  and  constriction  of  the  pupil,  the  erection  of 
the  hairs,  and  the  changes  in  the  caliber  of  the  bloodvessels. 
All  these  reactions  are  not  dominated  by  the  will,  and  are, 
therefore,  reflex  in  character. 

Perception  Reflexes. — The  importance  which  is  usually 
attached  to  those  reflexes  which  are  evoked  with  the  aid 
of  the  spinal  cord,  may  have  led  us  to  believe  that  this 
structure  is  practically  the  only  one  mediating  these  reactions. 
This  is  by  no  means  true,  because  many  impulses  subserving 
reflex  action  never  enter  the  spinal  cord,  but  remain  con- 
fined to  the  sympathetic  system  or  certain  tracts  of  the 
basal  portions  of  the  brain.  Thus,  it  is  possible  to  elicit  a 
secretion  of  gastric  juice  by  local  stimulation  even  after  the 
stomach  has  been  separated  from  the  central  nervous  system 
by  the  division  of  all  its  connecting  paths.  Very  similar 
results  may  be  obtained  with  the  intestine  and  urinary  or- 
gans. These  facts  prove  that  the  autonomic  nervous  system 
is  in  possession  of  many  local  reflex  centers  which  are  capable 
of  acting  independently  of  central  parts  and  require  the 
latter  only  when  a  correlation  of  function  is  essential. 

Several  of  the  reflexes  afore-mentioned  actually  involve 
tracts  and  centers  which  might  rightly  be  said  to  belong  to 
the  cerebrum.  But,  since  these  impulses  are  not  controlled 
by  volition,  they  must  nevertheless  give  rise  to  simple  reflex 
acts.  In  fact,  several  reflexes  must  first  be  qualified  by  the 
element  of  perception  before  they  can  attain  their  full 
development.  Thus,  it  has  been  mentioned  above  that  a 
secretion  of  saliva  and  gastric  juice  usually  follows  the 
reception  of  impressions  of  smell,  taste,  and  sight  without 
local  stimulation  of  these  glands.  These  reflexes  embodying 
a  distinct  psychic  element,  are  known  as  perception  or  asso- 
ciation reflexes.  Likewise,  the  entrance  of  a  larger  particle 
of  dust  into  the  conjunctival  sac  calls  forth  a  copious  secre- 
tion of  lacrimal  fluid.  This  is  a  pure  reflex.  But,  a  copious 
flow  of  lacrimae  may  also  result  in  consequence  of  certain 
emotional  concepts.  In  the  latter  case,  this  reaction  pos- 
sesses the  character  of  a  perception-reflex. 


CHAPTER  XXXII 
THE    BRAIN 

The  General  Arrangement  of  the  Brain. — The  brain 
occupies  the  cavity  of  the  cranium,  which  is  formed  by  the 
union  of  eight  plate-like  bones:  namely,  the  frontal,  two 
parietal,  occipital,  two  temporal,  sphenoid,  and  ethmoid. 
It  consists  of  several  parts,  the  largest  of  which  are  the  fore- 
brain  or  cerebrum  and  the  hindbrain  or  cerebellum.  Upon 
the  under  surfaces  of  these  structures  are  found  several 
smaller  masses  of  nerve  tissue,  namely,  the  optic  tract, 
pituitary  body,  corpora  quadrigemina,  and  the  pons  and 
medulla  oblongata.  The  two  structures  mentioned  last 
constitute  the  connecting  bridge  between  the  cerebrum  and 
the  spinal  cord.  They  are  arranged  in  the  form  of  a  stem 
around  which  the  higher  parts  are  moulded. 

The  enveloping  membranes  of  the  brain  bear  the  same 
names  and  present  the  same  general  appearance  as  those 
investing  the  spinal  cord.  Directly  within  the  inner  plate 
of  the  cranial  bones  lies  the  dura  mater,  and  next  to  it,  the 
arachnoid.  Between  the  latter  and  the  pia  mater  are  found 
numerous  spaces  which  frequently  acquire  a  considerable 
size  and  are  filled  with  a  lymph-like  fluid,  bearing  the  char- 
acteristics of  the  general  cerebro-spinal  liquid.  The  pia 
follows  the  surface  of  the  brain  very  closely,  dipping  into  all 
its  furrows,  while  the  dura  and  arachnoid  do  not. 

The  central  nervous  system  of  the  higher  animals  is  not  a 
solid  mass  of  tissue,  but  gives  lodgment  to  numerous  spaces 
and  channels  which  are  filled  with  cerebro-spinal  fluid.  This 
system  begins  in  front  with  the  lateral  ventricles,  the  walls  of 
which  are  formed  by  the  cerebrum  or  forebrain.  As  is  indi- 
cated in  Fig.  119,  these  spaces  communicate  with  the  third 
ventricle  which  is  situated  within  the  narrowed  part  con- 
stituting the  "'tween-brain."  Then  follows  the  aqueduct 

320 


THE   BRAIN  321 

of  Sylvius  within  the  mid-brain,  and  lastly,  the  fourth  ven- 
tricle surrounded  by  the  cerebellum  or  hindbrain.  The 
channel  then  narrows  into  a  tube  which  traverses  the  entire 
spinal  cord  about  midpoint  between  its  anterior  and  posterior 
fissures. 

All  these  spaces  contain  a  lymph-like  fluid  which  is  known 
as  cerebro-spinal  liquid  and  originates  in  a  glandular  struc- 
ture of  the  cerebral  ventricles,  called  the  choroid  plexus. 
The  composition  of  this  medium  is  very  similar  to  that  of 


FIG.   119. — Diagrammatic  median  longitudinal  section  of  a  mammalian 
brain.     (Edinger,) 

the  fluid  filling  the  subdural  and  subarachnoid  spaces. 
Sufficient  evidence  is  at  hand  to  show  that  all  these  passages 
communicate  with  one  another,  and  that  the  cerebro-spinal 
fluid  is  finally  drained  off  through  the  lymphatic  ducts  of  the 
head  and  the  large  veins  of  the  cranium.  The  latter  receive 
at  frequent  intervals  minute  nipple-like  projections  from  the 
arachnoid  which  are  termed  Pacchionian  bodies.  These 
small  membranous  saccules  are  suspended  in  the  bloodstream, 
thereby  bringing  the  subarachnoidal  fluid  into  diffusion 
relation  with  the  venous  blood.  While  these  projections 
serve  as  natural  outlets  for  the  lymph,  the  process  by  means 
of  which  the  latter  enters  the  blood  is  not  one  of  simple 
filtration. 

The  Simple  Brain. — In  order  to  be  able  to  obtain  a  concise 
idea  regarding  the  structure  of  the  complex  brain  of  the 
mammals,  it  seems  best  to  initiate  this  subject-matter  with  a 

21 


322 


THE   NERVOUS    SYSTEM 


brief  study  of  the  simple  brain  of  the  frog  (Fig.  120).  The 
cranial  cavity  of  this  animal  is  situated  well  forward  between 
the  eyes.  On  opening  it  two  white,  hemispherical  masses 
of  nerve  tissue  are  brought  into  view  which  jointly  con- 
stitute the  cerebrum.  Farther  forward  lie  two  long,  bul- 
bular  masses  of  tissue  which  are  termed  the  olfactory  lobes. 
Attention  has  already  been  called  to  the  fact  that  the  move- 
ments of  these  animals  are  controlled  chiefly  by  the  sense  of 


OLFACTORY  NERVE 
..OLFACTORY  LOBE 

.CEREBRUM 

THALAMENCEPHAL°N 
-OPTIC  LOBE 
-FOURTH  VENTRICLE 

LONG.  FISSURE  OF 
4™  VENTRICLE 


FIG.  120. — The  brain  of  the  frog. 

smell.  For  this  reason,  we  find  that  their  cerebrum  is  very 
largely  taken  up  by  the  " association  center"  pertaining  to 
this  sense. 

The  'tweenbrain,  midbrain  and  hindbrain  of  the  frog  and 
allied  animals  present  a  very  rudimentary  development:, 
which  again  is  in  entire  agreement  with  their  mode  of  life. 
They  move  very  sluggishly  and  progress  principally  along 
straight  lines  without  executing  rotary  motions.  Since  the 
cerebellum  gives  rise  to  co-ordinated  muscular  movements, 


THE   BRAIN  323 

it  will,  therefore,  be  seen  that  it  need  not  be  highly  developed 
in  these  animals.  The  reverse  condition  prevails  in  the 
birds,  while  the  mammals  occupy  in  this  regard  an  inter- 
mediary position. 

Back  of  the  'tweenbrain  lie  two  grayish,  rounded  masses 
of  nerve  tissue  which  are  designated  as  the  optic  lobes,  and 
back  of  these,  the  medulla  oblongata  and  spinal  cord.  The 
optic  lobes  are  highly  developed  in  these  animals.  They 
aid  in  controlling  motor  actions,  and  correspond,  therefore, 
to  the  corpora  quadrigemina  of  the  mammals.  In  the 
higher  animals  these  bodies  are  not  very  conspicuous, 
because  their  function  has  been  transferred  in  a  large  measure 
to  other  parts  of  the  nervous  system. 

The  Complex  Brain. — The  functional  development  of  the 
brain  requires  an  increase  not  only  in  its  size  and  weight  but 
also  in  its  complexity.  Having  reached  the  group  of  the 
mammals,  the  first  factor  becomes  of  lesser  importance 
than  the  second,  because  it  is  readily  noted  that  the  brains 
of  neighboring  species  then  frequently  possess  practically 
the  same  weight,  although  their  functional  capacity  is 
decidedly  different.  Any  additional  mental  power  is  finally 
gained  by  rendering  the  brain  more  uneven.  Numerous 
furrows  or  sulci  are  developed  in  its  outer  zone  which  sub- 
divide its  external  surface  into  numerous  long  and  narrow 
convolutions,  usually  pursuing  a  course  from  before  backward. 
The  weight  of  the  cerebrum  amounts  to  1500  grams  in  the 
human  male  and  to  1350  grams  in  the  human  female.  Its 
weight  increases  rapidly  up  to  about  the  fifth  year,  but 
remains  practically  stationary  after  the  eighteenth  year. 
Only  the  whale  and  elephant  have  a  heavier  brain  than  man, 
although  the  intelligence  of  these  animals  by  no  means 
equals  that  of  man. 

This  increased  complexity  of  the  higher  brain  signifies 
that  it  gives  lodgment  to  a  much  larger  number  of  neurones 
than  the  lower,  these  elements  being  required  for  the  psychic 
processes  of  reflection,  intelligence,  and  volition.  Thus, 
the  human  brain  has  lost  much  of  that  kind  .of  nervous 
material  which  accomplishes  the  ordinary  reflex  interchanges. 
Instead,  it  has  acquired  a  certain  number  of  nervous  units 


324 


THE    NERVOUS    SYSTEM 


in  which  those  associations  arise  that  impart  a'  distinct 
psychic  quality  to  their  more  fundamental  processes. 

As  has  been  stated  above,  this  additional  number  of 
neurones  finds  sufficient  space  in  the  brain  by  virtue  of  the 
folded  condition  of  its  surface,  every  additional  furrow 
serving  as  a  depository  for  these  cells;  This  arrangement 
calls  to  our  minds  the  manner  in  which  the  area  of  the  alveolar 
surface  of  the  lungs  is  increased  to  augment  diffusion.  To 


S.  precentralii  nftrior 

8.  cmtralu  (Roland*) 

S.  poOcentratit  inferior 


tamta  anl.  *ort«m<aK» 


Kama  pott  of  Sylrian  f 


FIG.  121. — Left  cerebral  hemisphere  from  the  lateral  aspect. 
(J.  Symington. ) 

begin  with,  these  organs  appear  in  the  form  of  two  separate 
pouches,  possessing  perfectly  smooth  inner  surfaces.  When, 
however,  the  constantly  increasing  metabolic  requirements 
of  these  animals  necessitate  a  more  ample  interchange  of  the 
gases,  the  formerly  single  air  spaces  are  subdivided  into  many 
by  thin  partitions  projecting  inward  from  their  walls.  These 
partitions  become  confluent  in  the  higher  forms,  so  that  each 
lung  is  subdivided  into  many  millions  of  minute  air-cells  or 
alveoli. 

The  Arrangement  and  Structure  of  the  Gray  and  White 
Matter. — Contrary  to  the  spinal  cord,  the  gray  matter  of  the 


THE  BEAIN 

B 


325 


sw 


w 


FIG.  122. — Cross-section  of  A,  cortex  of  frontal  convolution;  B,  cortex 
of  posterior  central  convolution;  C,  cortex  of  middle  occipital  convolution. 
N,  neuroglia;  SW,  superficial  white  fibers;  M,  molecular  layer;  P, 
pyramidal  cells;  W^,  second  transverse  fibers;  Wa,  third  transverse 
fibers;  Po,  polygonal  cells;  W,  white  matter. 


326  THE    NERVOUS    SYSTEM 

brain  is  situated  outside  the  white  matter.  Thus,  any  cross- 
section  of  the  cerebrum  or  cerebellum  invariably  presents  an 
outer  zone  of  a  grayish-red  substance  situated  next  to  the 
pia  mater,  and  an  inner  core  of  white  matter.  This  arrange- 
ment leads  us  to  suspect  that  the  cell-bodies  of  the  various 
neurones  lie  directly  below  the  surface  of  the  brain,  while 
the  fibers  forming  their  afferent  and  efferent  connections 
occupy  a  position  centrally  to  them. 

If  a  histological  preparation  of  the  cerebrum  is  placed  under 
the  high  power  of  a  microscope,  it  will  be  seen  that  its  cortical 
substance  consists  of  a  supporting  framework  of  neuroglia 
tissue,  in  which  are  embedded  the  cell-bodies  and  central 
processes  of  a  large  number  of  neurones.  These  cell-bodies 
are  arranged  in  several  layers  and  possess  in  most  instances 
a  pyramidal  shape.  Their  tips  are  directed  outward,  while 
their  broad  basal  portions  are  turned  inward  and  send  a  well 
defined  axone  into  the  underlying  white  matter. 

A  general  idea  regarding  the  minute  structure  of  the  cere- 
bral cortex  may  be  obtained  from  Fig.  122.  As  has  been 
emphasized  above,  its  most  conspicuous  elements  are  the 
pyramidal  cells  of  the  motor  realm.  It  should  be  remem- 
bered, however,  that  this  histological  picture  changes  con- 
siderably in  the  different  regions  of  this  organ.  Thus,  it 
should  be  noted  first  of  all  that  the  thickness  of  the  human 
cerebrum  varies  from  4  mm.  to  2  mm.,  and  furthermore, 
while  its  posterior  realm  is  characterized  by  a  prominent 
layer  of  granular  cells,  its  upper  or  parietal  region  embraces 
a  zone  of  very  large  pyramidal  cells  which  are  known  as  the 
cells  of  Betz.  These  cells  are  of  particular  importance, 
because  they  generate  those  impulses  which  give  rise  to  the 
contractions  of  the  skeletal  muscles.  Their  axones  enter 
the  cerebral  white  matter,  and  finally  descend  in  the  spinal 
cord  through  the  anterior  and  crossed  pyramidal  tracts, 
two  typical  efferent  paths  of  the  projection-system.  It  will 
be  shown  later  that  all  these  fibers  cross  to  the  opposite  side, 
this  crossing  being  effected  either  in  the  medulla  oblongata 
or  in  the  spinal 'cord  itself. 


CHAPTER  XXXIII 
THE  CEREBRUM 

The  Different  Regions  of  the  Cerebral  Cortex. — The  cere- 
brum is  composed  of  two  halves  or  hemispheres  which  are 
separated  from  one  another  by  a  deep  furrow,  termed  the 
great  longitudinal  fissure.  The  floor  of  this  cleft  is  formed 
by  a  narrow  bridge  of  white  matter  connecting  the  two 
hemispheres  with  one  another.  It  is  designated  as  the 
corpus  callosum.  From  the  little  brain  or  cerebellum,  the 
cerebrum  is  separated  by  a  deep  horizontal  depression  which 
gives  lodgment  to  a  strong  septum  of  dura  mater,  the  tentor- 
ium  cerebelli.  The  outer  convex  surfaces  of  the  cerebral 
hemispheres,  as  well  as  their  flat  median  surfaces  adjoining 
the  longitudinal  fissure,  present  numerous  grooves  or  sulci 
which  subdivide  these  areas  into  many  smaller  ones,  possess- 
ing as  a  rule  a  long  and  narrow  shape.  Two  of  these  fissures 
are  very  clearly  outlined,  their  conspicuousness  enabling  us  to 
employ  them  as  general  landmarks  in  localizing  the  functions 
of  this  organ.  They  are  known  as  the  fissure  of  Rolando  and 
fissure  of  Sylvius.  The  former  is  situated  about  the  middle  of 
the  outer  surface  of  the  hemisphere,  and  pursues  an  oblique 
course  forward  and  downward  from  the  longitudinal  fissure, 
beginning  about  half  an  inch  behind  the  mid-point  between 
the  globella  and  the  occipital  protuberance.  The  latter 
begins  at  the  base  of  the  brain  at  a  distance  of  about  5  cm. 
behind  the  external  angular  process,  and  runs  outward  to  the 
external  surface  of  the  hemisphere. 

These  fissures,  together  with  the  parieto-occipital  groove, 
divide  the  external  surface  of  each  cerebral  hemisphere  into 
five  lobes:  namely,  the  frontal,  parietal,  occipital,  temporo- 
sphenoidal,  and  island  of  Reil.  The  frontal  lobe  is  situ- 
ated in  front  of  the  fissure  of  Rolando  and  above  the  fissure 
of  Sylvius.  Back  of  the  fissure  of  Rolando  lies  the  parietal 

327 


328  THE    NERVOUS    SYSTEM 

lobe,  its  posterior  boundary  being  formed  by  the  parieto- 
occipital  fissure,  and  its  lower  boundary  by  the  horizontal 
limb  of  the  fissure  of  Sylvius.  The  posterior  pole  of  each 
hemisphere  is  formed  by  the  occipital  lobe.  Below  the  fissure 
of  Sylvius  lies  the  temporal  lobe,  which  occupies  practically 
the  entire  middle  fossa  of  the  skull.  The  island  of  Reil  is 
hidden  from  the  view,  because  it  is  situated  in  the  fissure  of 
Sylvius  at  the  base  of  the  brain. 

The  most  important  fissure  upon  the  inner  or  median 
surface  of  the  cerebral  hemisphere  is  the  calloso-marginal. 
It  pursues  a  course  upward  and  backward,  parallel  to  the 
corpus  callosum.  The  calcarine  fissure  is  of  importance  to  us 
at  this  time,  because  it  indicates  the  location  of  the  center 
of  sight.  This  groove  eventually  joins  the  parieto-occipital 
fissure. 

The  fibers  emerging  from  these  cortical  areas  of  gray  sub- 
stance, make  three  principal  connections:  namely,  (a)  with 
other  areas  of  the  same  hemisphere;  (6)  with  areas  in  the 
opposite  hemisphere;  and  (c)  with  the  spinal  cord  and  distant 
parts.  The  first  form  the  so-called  association  system,  the 
second,  the  commissural  system,  and  the  third,  the  projection 
system.  Notice  should  also  be  taken  at  this  time  of  the  fact 
that  two  important  masses  of  gray  matter  are  situated  below 
the  cortex  and  directly  in  the  path  of  the  projection  fibers. 
They  are  the  optic  thalami  and  the  sthate  bodies.  The 
former  lie  one  to  each  side  of  the  third  ventricle  and  below 
the  lateral  ventricle,  thus  forming  a  deposit  of  gray  matter 
upon  the  upper  surface  of  each  cms  cerebri.  The  crus,  as  has 
been  stated  above,  represents  the  most  compact  portion  of 
the  bundle  of  fibers  passing  away  from  the  cerebral  cortex 
and  striving  to  attain  the  narrow  point  of  exit  afforded  them 
in  reaching  the  medulla.  The  striate  bodies  are  situated 
somewhat  in  front  of  the  optic  thalami,  and  form  a  promi- 
nence upon  the  floor  and  wall  of  the  lateral  ventricle^ 
Each  striate  body,  however,  embraces  two  parts,  one  lying 
directly  in  front  of  the  thalamus  in  the  position  just  indicated, 
and  one  at  the  side  of  this  mass  of  gray  substance. 

The  Removal  of  the  Cerebrum. — In  analyzing  the  func- 
tions of  the  different  segments  of  the  central  nervous  system, 


THE    CEREBRUM  329 

use  is  generally  made  of  several  different  methods  which 
may  be  grouped  under  the  following  headings :  (a)  removal  or 
enucleation  of  the  part  under  observation;  (6)  stimulation  of 
its  cortical  gray  matter  or  underlying  white  matter;  (c) 
tracing  of  the  fibers  connecting  it  with  other  structures,  and 
(d)  observation  of  the  symptoms  accompanying  inflammatory 
and  destructive  lesions  of  its  substance.  Of  particular 
importance  is  the  study  of  clinical  cases,  because  it  permits  us 
to  substantiate  the  conclusions  drawn  from  the  results  of 
experimental  lesions  in  animals.  Thus,  while  it  is  a 
comparatively  simple  matter  to  note  losses  of  motor 
function  in  animals,  it  is  usually  impossible  to  detect 
disturbances  of  the  sensations,  because  in  the  latter  case  we 
are  almost  wholly  dependent  upon  the  description  of  sensory 
impressions,  which  an  animal  cannot  give.  For  this  reason, 
the  physiologist  must  gain  his  information  in  most  instances 
from  an  analysis  of  the  disturbances  following  certain  patho- 
logical lesions  of  the  nervous  system  of  man.  Such  lesions 
are  by  no  means  uncommon  and  usually  result  in  consequence 
of  hemorrhages,  injuries,  and  the  growth  of  tumors. 

Let  us  see  first  of  all  what  changes  are  evoked  by  the  re- 
moval of  the  cerebral  hemispheres.  Attention  has  already 
been  called  in  Chapter  XXII  to  the  fact  that  this  procedure 
destroys  the  psychic  life  of  the  animal,  whether  simple  or 
complex.  Accordingly,  it  may  be  surmised  that  this  struc- 
ture is  the  seat  of  associative  memory.  This  brief  statement 
may  be  greatly  amplified  by  studying  the  behavior  of  anyone 
of  those  animals  whose  general  movements  remain  practically 
the  same  after  the  loss  of  this  part  of  the  nervous  system. 
Thus,  it  may  be  gathered  from  the  behavior  of  the  shark  after 
the  removal  of  its  cerebrum  that  the  fish  are  only  slightly 
affected  by  the  loss  of  this  structure.  They  reveal  the  same 
power  of  movement  as  normal  animals,  although  invariably 
tending  to  assume  a  rather  continuous  position  of  rest  which 
is  exchanged  for  one  of  activity  only  upon  stimulation. 
But  when  made  to  move,  their  motor  reactions  exhibit  a 
perfectly  normal  character. 

Very  similar  changes  are  presented  by  the  decerebrated 
frog.  Its  usual  attitude  is  one  of  inactivity,  although  it 


330  THE    NERVOUS    SYSTEM 

maintains  its  posture  so  well  that  it  cannot  easily  be  detected 
when  allowed  to  mingle  with  a  number  of  other  perfectly 
normal  frogs.  One  way  of  establishing  its  identity  is  to  pass 
the  hand  over  the  aquarium.  The  normal  frogs  will  then 
make  muscular  efforts  to  escape,  because  although  not  able 
to  form  distinct  visual  concepts,  such  a  movement  is  received 
by  them  as  a  "shadow"  possessing,  possibly,  certain  injuri- 
ous consequences.  Since  the  actions  of  the  decerebrated 
frog  are  no  longer  controlled  by  associations,  it  retains  its 
position  even  in  the  direct  path  of  danger. 

Very  similar  conclusions  may  be  drawn  from  the  changes 
taking  place  in  the  "act  of  croaking"  after  the  cerebrum  has 
been  removed.  Ordinarily,  the  frog  produces  its  character- 
istic sounds  only  when  the  conditions  in  the  pond  are  perfect. 
Subsequent  to  the  loss  of  the  cerebrum,  however,  this  for- 
merly associative  act  assumes  the  character  of  a  simple  reflex. 
Thus,  it  may  now  be  elicited  at  any  time  by  simple  stimula- 
tion, such  as  may  be  produced  by  touching  the  back  or  sides 
of  the  frog.  In  the  absence  of  the  cerebrum  these  tactile 
impacts  evoke  a  contraction  of  the  resonating  pouches 
without  being  first  acted  upon  by  the  higher  centers.  Voli- 
tion has  been  definitely  removed  from  this  reaction. 

A  frog  deprived  of  its  cerebrum  jumps  and  swims  nor- 
mally and  rights  itself  when  placed  upon  its  back.  Its  bal- 
ancing movements  reveal  a  perfectly  normal  character.  It 
reacts  to  stimuli  applied  to  its  nasal  mucosa,  and  avoids 
obstacles.  Furthermore,  its  digestive  processes  are  not 
impaired,  although  it  will  not  eat  spontaneously.  But  since 
its  simple  as  well  as  automatic  reflex  mechanisms  are  in 
perfect  condition,  an  animal  of  this  kind  may  be  kept  alive 
for  many  years  provided  it  is  placed  in  proper  surroundings 
and  is  fed  from  time  to  time.  If  the  food  is  placed  in  its 
mouth,  it  is  swallowed  and  digested  in  a  normal  manner. 

Practically  the  same  behavior  is  manifested  by  birds  after 
they  have  been  deprived  of  their  cerebral  hemispheres. 
Thus,  the  decerebrated  pigeon  continues  in  an  inactive  state 
for  long  periods  of  time,  but  may  be  made  to  move  at  any 
time  by  stimulation.  If  tossed  into  the  air,  it  will  fly,  but 
not  for  any  length  of  time.  Moreover,  in  alighting  it  will 


THE    CEREBRUM  331 

select  practically  any  perch  and  even  one  that  may  prove 
injurious  to  it.  As  has  been  stated  above,  the  removal  of 
the  cerebrum  destroys  associative  memory.  In  consequence 
of  this  loss  the  animal  is  no  longer  able  to  profit  by  experience. 
Thus,  the  decerebrated  pigeon  will  not  take  food  even  when 
placed  directly  in  front  of  it,  but  if  fed  and  taken  proper 
care  of,  it  will  live  practically  as  long  as  a  normal  one.  Its 
life,  however,  is  that  of  a  simple  reflex  animal. 

Very  similar  results  have  been  obtained  in  cats,  dogs,  and 
monkeys.  In  these  animals,  however,  the  removal  of  the 
cerebrum  is  a  much  more  difficult  task  than  in  the  lower 
forms,  because  their  cerebral  white  matter  embraces  a 
number  of  subcortical  and  basal  masses  of  gray  matter,  the 
destruction  of  which  gives  rise  to  certain  motor  disturbances 
of  a  general  character.  For  this  reason,  the  enucleation  of 
the  cerebrum  in  mammals  should  be  restricted  to  its  cortical 
portion  and  adjoining  white  matter.  It  may  suffice  to  refer 
at  this  time  to  three  decerebrate  dogs,  the  behavior  of  which 
was  carefully  studied  for  as  long  a  time  as  eighteen  months 
after  the  operation.  These  animals  began  to  move  about 
within  a  few  days  after  the  operation  and  even  walked  across 
inclined  planes.  They  avoided  obstacles  and  reacted  to 
diverse  sensory  stimuli  by  snarling,  barking,  and  the  erection 
of  the  ears.  Their  motor  reactions,  however,  were  not 
controlled  by  associations.  The  greatest  part  of  each  day 
was  spent  by  them  in  absolute  rest.  They  did  not  seek  food, 
but  finally  took  it  when  it  was  placed  directly  in  front  of 
them. 

The  aforesaid  general  deductions  pertaining  to  the  function 
of  the  cerebrum  are  also  applicable  to  man.  We  are  re- 
minded at  this  time  of  the  disturbances  accompanying  general 
paresis  (softening  of  the  brain)  which  condition  most  closely 
resembles  that  established  in  the  lower  animals  by  the  ex- 
perimental removal  of  the  cerebrum.  The  power  to  acquire 
new  memory  reactions  is  lost  and  even  the  stored  memory 
concepts  gradually  pass  out  of  existence,  until  the  formerly 
intelligent  behavior  of  the  person  closely  approaches  that 
of  a  reflex  animal. 

Nature  also  supplies  us  constantly  with  many  very  instruc- 


332  THE    NERVOUS    SYSTEM 

tive,  although  deplorable,  cases  of  partial  destruction  of  the 
central  nervous  system.  We  are  reminded  at  this  time  of  the 
lacerations  of  the  cerebrum  by  bullets  and  other  solid  bodies 
as  well  as  of  the  destruction  of  certain  areas  of  this  organ  by 
hemorrhages,  tumors,  and  inflammations.  The  case  most 
frequently  cited  in  the  literature  is  that  of  a  stone-cutter 
who  permitted  an  iron  rod  to  slip  into  a  borehole,  into  which 
he  had  previously  placed  a  certain  amount  of  dynamite. 
The  rod  traversed  his  skull  from  below  upward,  tearing  away 
a  very  considerable  portion  of  his  frontal  lobes.  Curiously 
enough,  this  severe  injury  was  followed  by  an  uneventful 
recovery.  Nothing  more  than  a  relatively  slight  retrogres- 
sive change  in  his  character  and  intelligence  could  be  noted 
subsequent  to  the  injury. 

Many  cases  of  inherited  absence  of  the  cerebrum  have  also 
been  recorded.  These  infants  showed  a  persistence  of  the 
spinal  reflexes,  and  particularly  those  of  mastication,  sucking, 
crying,  and  grasping.  Of  particular  interest  is  the  case  of  a 
child  whose  cerebrum  was  destroyed  by  disease  when  about 
two  years  of  age.  Two  years  later  it  was  found  at  autopsy 
that  the  cranial  cavity  was  filled  with  fluid  (hydrocephalus) . 
During  the  interim  the  child  lay  passive  in  its  bed  as  though 
sleeping,  giving  no  signs  of  intelligence  or  initiative  purpose- 
ful movements.  Its  spinal  and  bulbar  reflexes,  however, 
were  well  preserved. 


CHAPTER  XXXIV 

THE    LOCALIZATION    OF    FUNCTION    IN    THE 
CEREBRUM 

The  Motor  Area. — The  cerebral  hemispheres  have  been 
regarded  as  the  material  seat  of  consciousness  since  antiquity. 
Thus,  it  was  believed  that  their  frontal  regions  subserve 
imaginative  qualities,  while  their  central  portions  give  rise  to 
intelligence,  and  their  posterior  realms  to  memory.  This 
localization  of  the  psychic  processes  in  different  regions  of  the 
cerebrum  has  been  made  thorough  use  of  by  the  originators 
of  the '" science"  of  phrenology.  It  was  proposed  by  them 
that  the  character,  behavior  and  accomplishments  of  men 
can  be  correlated  with  the  contours  of  the  brain,  and  that  the 
varying  development  of  the  different  segments  of  the  latter 
evokes  corresponding  changes  in  the  general  configuration 
of  the  skull.  Thus,  it  was  conjectured  that  a  musician  must 
present  an  excessive  development  of  the  temporal  lobes  and 
an  unusual  prominence  of  the  corresponding  regions  of  the 
cranium.  For  similar  reasons,  a  person  skilled  in  sketching 
and  painting  was  supposed  to  develop  in  time  an  extraordi- 
nary prominence  of  the  occipital  areas  of  the  cranium. 

While  in  the  light  of  our  present-day  knowledge  a  mode- 
rated conception,  of  this  kind  can  be  successfully  defended, 
the  phrenologists  were  not  satisfied  with  simple  statements, 
but  extended  their  localization  practically  to  all  human  traits 
and  endeavors.  In  accordance  with  their  chiefly  hypotheti- 
cal doctrines  the  skull  is  beset  with  an  almost  unlimited 
number  of  " bumps"  which  are  employed  by  them  to  analyze 
the  present  and  future  qualities  and  possibilities  of  the 
person.  Accordingly,  this  conception  was  exploited  in 
a  commercial  way  by  advising  the  person  what  calling  in  life 
he  should  follow  in  order  to  make  the  best  use  of  his  cerebral 
peculiarities.  These  often  far-fetched  inferences  derived 

333 


334 


THE    NERVOUS    SYSTEM 


from  imaginary  details  of  the  outline  of  ,the  cranium,  gradu- 
ally brought  phrenology  into  lasting  disrepute 

Opposed  to  this  view  embodying  the  principle  that  the 
cerebrum  is  a  plurality  of  organs,  is  the  one  which  holds  that 
this  organ  is  absolutely  homogeneous  in  its  structure  and 
function,  and  gives  rise  to  consciousness  in  the  form  of  an 
indivisible  product.  During  the  years  of  1825  to  1850, 
however,  Broca  and  others  collected  certain  evidence  which 


MOTOR  AREA 


SEMSORV  AREA 


AUDITORY  AREA 
VISUAL   AREA 


EftEBEl-LAJM 


MEDULLA 
CORD 


FIG.  123. — Cerebral  localization  (external  surface). 

showed  that  speech  is  controlled  by  a  circumscribed  area 
situated  in  the  front  part  of  the  left  cerebral  hemisphere. 
Somewhat  later  Jackson  proved  that  the  muscular  spasms 
characterising  epilepsy  result  in  consequence  of  an  excitation 
of  the  cerebral  cortex.  Then  followed  the  observations  of 
Fritsch  and  Hitzig,  which  demonstrated  that  the  surface 
of  the  cerebral  cortex  is  irritable.  These  investigators 
demonstrated  that  the  electrical  excitation  of  a  definite  realm 
of  the  cerebrum  gives  rise  to  contractions  of  certain  skeletal 
muscles.  For  this  reason,  the  term  motor  area  has  been 
applied  to  this  circumscribed  region  of  the  cortex. 


LOCALIZATION   OF   FUNCTION    IN   THE    CEREBRUM      335 

In  the  cat,  dog,  and  monkey  the  motor  area  occupies  the 
region  in  front  and  behind  the  fissure  of  Rolando,  while  in 
the  apes  and  man  it  embraces  solely  the  anterior  central 
convolution  of  each  side.  If  either  area  is  now  more  care- 
fully mapped  out,  it  will  be  found  that  the  cellular  constitu- 
ents of  its  upper  portion  situated  next  to  the  horizontal 
fissure,  govern  the  muscular  reactions  of  the  trunk  and  legs. 
At  a  somewhat  lower  level  are  found  those  ganglion  cells 
which  innervate  the  muscles  of  the  arms,  and  at  a  level 
opposite  the  lower  end  of  the  aforesaid  fissure,  those  cells 
which  control  the  movements  of  the  face.  Thus,  it  may  be 
stated  that  the  motor  field  of  the  cerebral  cortex  is  divided 
into  a  right  and  a  left  area,  and  each  area  in  turn  into  three 
minor  realms.  In  fact,  by  employing  a  special  set  of  elec- 
trodes, it  is  possible  to  evoke  even  more  minute  movements, 
such  as  flexion  and  extension  of  the  fore-  and  hind-limbs,  as 
well  as  movements  of  the  muscles  of  the  tongue  and  eyes. 

It  will  be  remembered  that  these  areas  contain  large, 
pyramidal  ganglion  cells  which  are  known  as  the  cells  of 
Betz.  It  has  already  been  stated  above  that  the  axones 
emerging  from  their  cell-bodies,  traverse  the  cerebral  white 
matter  and  enter  the  spinal  cord,  where  they  form  synapses 
with  efferent  neurones  of  the  second  order.  It  is  to  be 
noted,  however,  that  all  these  fibers  eventually  gain  the 
opposite  side  of  the  cord  either  by  crossing  in  large  numbers 
in  the  medulla  oblongata  or  more  gradually  as  they  descend 
through  the  spinal  efferent  tracts.  The  fibers  effecting 
their  crossing  in  the  medulla,  form  a  very  conscript  path  to 
which  the  name  of  pyramidal  decussation  has  been  given. 
This  decussation  of  the  efferent  fibers  of  the  projection 
system  lies  in  relation  with  a  similar  crossing  of  the  sensory 
fibers  of  the  same  system. 

The  fact  that  the  nerve  cells  of  the  motor  areas  control 
the  actions  of  the  muscles  on  the  opposite  side  of  the  body, 
should  make  it  evident  to  us  that  an  injury  to  either  one 
of  them  must  result  in  a  paralysis  involving  the  opposite 
musculature,  excepting  the  muscles  of  respiration.  This 
condition  which  is  known  as  hemiplegia,  is  brought  about 
at  times  by  a .  stroke  upon  the  f ronto-parietal  region  of 


336  THE    NERVOUS    SYSTEM 

the  cranium  and  consequent  escape  of  blood  from  one  of 
the  dural  vessels  into  the  neighboring  cerebral  cortex.  The 
blood  may  also  accumulate  upon  or  below  the  dura  mater 
and  exert  a  direct  pressure  upon  the  underlying  motor  area. 

In  this  connection  brief  reference  should  also  be  made  to 
that  form  of  epilepsy  which  finds  its  origin  in  a  mechanical 
excitation  of  the  cerebral  cortex.  It  is  common  knowledge 
that  persons  afflicted  with  this  disease,  suffer  from  periodic 
attacks  of  violent  muscular  spasms,  each  attack  being  ini- 
tiated by  tremors  and  twitchings  of  a  particular  muscle  or 
groups  of  muscles.  The  neighboring  muscles  are  involved 
gradually  until  the  body  as  a  whole  is  retained  in  an  almost 
continuous  state  of  spasmodic  contraction.  These  seizures 
last  for  varying  periods  of  time,  and  are  repeated  as  a  rule  at 
irregular  intervals.  In  many  instances,  these  attacks  are 
the  direct  result  of  cerebral  excitation.  A  projecting  piece 
of  bone,  the  remnant  of  an  earlier  fracture  of  the  cranium, 
may  press  upon  the  motor  area,  or  an  extravasation  of  blood 
may  have  taken  place  in  consequence  of  an  injury  to  the 
head  which  gives  rise  to  a  mechanical  irritation  of  the  neigh- 
boring gray  matter.  If  traceable  to  a  local  cause  of  this 
kind,  the  epileptic  seizures  will  cease  after  the  exciting  agent 
has  been  removed  by  operation. 

The  Body-sense  Area. — The  impulses  from  the  cutaneous 
sense-organs  are  relayed  to  that  portion  of  the  cerebral  cortex 
which  is  situated  directly  behind  the  fissure  of  Rolando. 
Besides  the  posterior  central  convolution,  this  area  also 
embraces  the  anterior  portion  of  the  parietal  lobe.  This 
localization  is  based  chiefly  upon  histological  data,  because 
the  sensory  fibers  ascending  through  the  spinal  cord,  form  a 
median  bundle  which  terminates  in  the  gray  matter  at  the 
base  of  the  brain,  whence  tertiary  fibers  conduct  these 
impulses  to  the  parietal  cortex.  Further  evidence  of  this 
character  has  been  derived  from  the  direct  stimulation  of  the 
surface  of  the  parietal  lobes  in  conscious  patients,  this  pro- 
cedure having  been  employed  in  these  instances  for  diag- 
nostic purposes.  These  patients  perceived  sensations  of 
touch  and  numbness  as  long  as  the  stimulation  remained 
confined  to  this  particular  area  of  the  cerebrum. 


LOCALIZATION    OF    FUNCTION    IN    THE    CEREBRUM        337 

Some  doubt  exists  as  yet  regarding  the  character  of  the 
sensations  mediated  by  this  area.  Since  pain  is  not  felt  as  a 
result  of  the  excitation  of  this  cortical  field,  it  is  commonly 
believed  that  the  associations  formed  here  pertain  solely  to 
touch,  temperature,  and  muscular  sensibility.  Obviously, 
if  these  sensations  are  intended  to  give  rise  to  motor  reac- 
tions, they  are  relayed  from  this  area  to  the  neighboring 


ENSORY  AREA 


CEREBELLUM 


FIG.  124. — Cerebral  localization  (median  surface). 

motor  realm,  and  by  way  of  the  corpus  callosum  to  the  cor- 
responding area  in  the  opposite  half  of  the  cerebrum. 

The  Center  for  Sight. — The  changes  noted  after  the  de- 
struction of  the  occipital  lobes,  either  experimentally  or  by 
pathological  processes,  have  proved  conclusively  that  the 
impulses  derived  from  the  retinae  are  associated  in  this 
particular  region  of  the  hemispheres.  The  area  most  directly 
concerned  is  the  one  situated  along  the  calcarine  fissure  on 
the  mesial  surface  of  each  hemisphere.  In  many  animals  the 
fibers  emerging  from  the  eyes  cross  over  in  their  entirety  to 
the  center  on  the  opposite  side.  Accordingly,  the  destruction 
of  one  occipital  lobe  would  cause  a  blindness  in  the  opposite 
22 


338  THE   NERVOUS    SYSTEM 

eye.  In  man,  however,  the  crossing  is  incomplete,  because 
the  fibers  from  the  outer  halves  of  the  retinae  always  remain 
on  the  same  side,  whereas  those  from  their  inner  halves  seek 
the  opposite  center.  Thus,  it  will  be  seen  that  the  ablation 
of,  say,  the  right  occipital  cortex  must  destroy  the  sight  in 
the  outer  half  of  the  right  eye  and  inner  half  of  the  left  eye. 

This  is  the  condition  of  half-blindness  or  hemianopia.  The 
yellow  spot  of  each  retina  which  is  its  most  sensitive  area,  is 
not  involved  in  this  blindness,  because  the  fibers  emerging 
from  it  find  representation  in  both  occipital  centers.  It  will 
also  be  seen  that  the  loss  of  the  receptive  power  of,  say,  the 
right  halves  of  the  retinae  must  blot  out  the  left  fields  of 
vision,  because  the  objects  situated  to  the  left  of  the  visual 
line  of  the  eye,  are  always  focalized  upon  the  right  side  of  its 
retina. 

The  Center  for  Hearing. — By  similar  means  it  has  been 
demonstrated  that  the  impulses  derived  from  the  organ  of 
Corti  of  the  internal  ear,  which  forms  the  receptor  for  the 
sound  waves,  are  relayed  through  the  eighth  cranial  nerve  and 
various  parts  of  the  brain-stem  until  they  arrive  in  the  cortex 
of  the  temporal  lobe.  The  area  most  directly  involved  is 
the  upper  convolution  of  this  lobe,  while  the  middle  and 
inferior  ones  are  set  aside  for  memory  concepts  pertaining  to 
sounds.  The  auditory  fibers  also  cross  in  large  numbers  to 
the  opposite  center,  so  that  the  destruction  of  either  tem- 
poral lobe  cannot  produce  permanent  deafness,  because  the 
function  of  this  area  is  then  taken  up  by  the  one  on  the 
opposite  side. 

The  Centers  for  Smell  and  Taste. — It  is  a  well  known  fact 
that  the  sense  of  smell  is  very  unequally  developed,  because 
some  animals,  such  as  the  dolphin  and  porpoise,  are  entirely 
deficient  in  olfactory  organs,  while  others,  such  as  the  dog, 
rat,  and  opossum,  show  a  high  development  of  this  sense. 
These  sensory  impulses  ascend  from  the  olfactory  area  in  the 
nasal  cavity  through  the  olfactory  nerve  into  the  anterior 
realm  of  the  cerebrum.  They  are  finally  relayed  to  the  distal 
limb  of  the  hippocampus,  where  they  find  psychic  representa- 
tion. The  sensations  of  taste  are  conveyed  to  the  cerebrum 
by  the  fifth,  ninth,  and  tenth  cranial  nerves.  They  are 


LOCALIZATION   OF   FUNCTION   IN   THE    CEREBRUM      339 

associated  in  the  hippocampal  area  near  the  anterior  end  of 
the  temporal  lobe. 

The  Centers  for  Speech  and  Writing — At  birth,  the 
cerebrum  is  practically  a  potential  organ  as  far  as  its  higher 
functions  are  concerned,  while  later  in  life  it  serves  as  a 
register  of  memory  concepts,  controlling  the  diverse  activities 
of  the  body.  Possibly  the  most  remarkable  acquired  faculty 
is  the  production  of  coherent  sounds  in  the  form  of  speech. 
While  practically  all  the  higher  animals  possess  the  power 
of  uttering  purposeful  sounds,  none  is  able  to  produce 
them  in  such  an  articulated  manner  as  man.  We  speak  in 
consequence  of  associations  derived  from  various  sensory 
impressions.  Thus,  the  centers  for  sight,  hearing,  taste, 
smell  and  touch  may  be  said  to  be  contributory  to  speech. 
The  concepts  formed  with  the  aid  of  these  centers,  are  again 
analyzed  by  a  group  of  ganglion  cells  which  are  usually  said 
to  be  situated  in  the  left  inferior  frontal  convolution.  This 
rather  definite  localization  of  the  speech  center,  however, 
leaves  out  of  consideration  the  fact  that  speech  is  a  combined 
faculty  and  cannot  really  be  restricted  to  this  narrow  sphere. 
Thus,  while  it  has  been  stated  by  Broca  that  the  destruction 
of  the  left  inferior  frontal  convolution  leads  to  a  loss  of 
speech  or  aphasia,  it  has  been  shown  in  more  recent  years  that 
this  result  may  also  follow  lesions  in  other  parts  of  the 
cerebrum,  chiefly  its  association  areas  for  sight  and  hearing. 

It  may,  therefore,  be  stated  in  a  very  general  way  that 
speech  is  made  possible  by  an  effector,  the  larynx,  the  differ- 
ent parts  of  which  are  under  the  control  of  certain  ganglion 
cells  situated  in  the  motor  area  of  the  cerebrum.  These 
cells,  together  with  the  corresponding  efferent  fibers  of  the 
vagus  nerve,  constitute  the  efferent  or  motor  circuit  of  the 
speech  mechanism.  Furthermore,  the  impulses  discharged 
by  these  cells  are  regulated  by  a  special  group  of  ganglion 
cells  forming  the  speech  center  proper.  The  activity  of  the 
latter  is  in  turn  controlled  by  others  of  the  same  character, 
constituting  the  association  realms  for  sight,  hearing,  etc. 
Because  of  these  close  connections,  a  loss  of  speech  or  aphasia 
must  follow  injuries  to  the  center  as  well  as  to  any  one  of  its 
contributory  association  realms.  The  term  aphasia,  how- 


340  THE    NERVOUS    SYSTEM 

ever,  signifies  that  the  lesion  is  situated  within  the  cerebrum. 
Consequently,  a  loss  of  speech  resulting  from  an  injury  to  the 
larynx  or  its  efferent  paths,  cannot  rightly  be  called  aphasia. 
Practically  the  same  statements  may  be  made  regarding 
the  nervous  mechanism  controlling  the  act  of  writing.  The 
location  of  its  efferent  or  motor  channel  need  not  be  con- 
sidered in  detail.  It  embraces  those  cells  and  fibers  of  the 
motor  area  which  control  the  muscular  actions  of  the  arms 
and  hands.  As  in  the  case  of  speech,  the  activation  of 
this  mechanism  is  dependent  upon  diverse  afferent  impulses 
and  their  proper  association.  Consequently,  the  act  of 
writing  is  a  combined  faculty  and  cannot  be  sharply  localized, 
although  sufficient  clinical  evidence  is  at  hand  to  show  that 
its  chief  center  lies  in  the  frontal  lobe  in  close  proximity  to 
the  psychic  area  for  speech. 


CHAPTER  XXXV 
THE  CEREBELLUM  AND  MEDULLA  OBLONGATA 

The  Structure  of  the  Cerebellum. — The  cerebellum  is 
situated  directly  below  the  posterior  lobes  of  the  cerebrum, 
but  is  separated  from  the  latter  by  a  dense  septum  of  dura 
mater,  called  the  tentorium.  It  occupies  the  posterior 
fossa  of  the  base  of  the  cranium,  and  measures  close  to  four 
inches  from  side  to  side,  about  two  and  one-half  inches  from 
before  backward,  and  two  inches  in  thickness  near  its  center. 
Its  average  weight  is  140  grams  or  about  one-tenth  of  the 
weight  of  the  entire  brain.  In  infants,  however,  its  size  is 
proportionately  much  smaller  than  in  adults.  Like  the 
cerebrum  it  consists  of  two  halves  or  hemispheres  which 
are  connected  with  one  another  by  a  median  portion,  the 
vermis.  The  latter  appears  to  be  the  fundamental  part  of 
this  organ,  because  in  the  lower  forms,  such  as  the  fishes  and 
reptiles,  the  hemispheres  attain  only  a  very  rudimentary 
size.  The  surface  of  this  structure  is  not  convoluted,  as  is 
that  of  the  cerebrum,  but  is  folded  by  numerous  transverse 
furrows  into  lamellae-like  strips,  bearing  a  close  resemblance 
to  the  sprigs  of  the  cedar  tree. 

If  a  cross-section  of  one  of  these  lamellae  is  placed  under 
the  ocular  of  a  microscope,  it  will  be  seqn  to  be  composed  of 
an  outer  zone  of  gray  matter  and  an  inner  zone  of  white 
matter.  The  gray  matter  consists  of  three  layers.  The 
innermost,  lying  in  relation  with  the  white  matter,  is  known 
as  the  nuclear  layer,  and  the  outermost,  as  the  molecular. 
Between  these  two  zones  is  placed  a  row  of  very  large  cells 
each  of  which  possesses  a  pear-shaped  cell-body  and  bushy, 
fan-shaped  dendrites.  These  very  characteristic  elements  of 
the  cerebellar  cortex  are  designated  as  the  cells  of  Purkinje. 
Their  axones  traverse  the  white  matter  and  form  connections 
with  the  central  nuclei  of  gray  matter,  situated  within  the 
vermis. 

341 


342  THE    NERVOUS    SYSTEM 

The  cerebellum  is  the  dorsal  outgrowth  of  the  brain-stem, 
and  lies  really  outside  the  main  paths  connecting  the  cere- 
brum and  spinal  cord.  It  is,  however,  closely  related  to  the 
aforesaid  parts  by  bridges  of  fibers,  forming  the  superior, 
middle  and  inferior  peduncles.  Possibly  the  most  striking 
structural  peculiarity  of  the  cerebellum  is  the  uniform 
arrangement  of  its  elements.  In  the  cerebral  cortex,  on  the 
other  hand,  we  have  noted  certain  differences  in  the  shape 


Uvula 

"  rali, 


Lobulus  postero-superior 
Lobulus  semilunaris  inferior 

Lobulus  gracilis  posterior 
\ppstgracili*  \  I 

\     Lobulus  gracilis  anterior 
Sulcus  komontalu  rnagnut  y 

Pyramis 

FIG.   125. — View  of  cerebellum  from  below.      (/.  Symington.) 

and  size  of  its  constituent  cells  as  well  as  in  their  position 
and  general  arrangement.  In  agreement  with  its  structural 
diversity,  we  have  observed  that  this  organ  consists  of  several 
parts  contributing  singly  to  consciousness.  Accordingly, 
the  fact  that  the  gray  matter  of  the  cerebellum  presents 
everywhere  the  same  structural  details,  may  rightly  prompt 
us  to  conclude  that  its  function  is  homogeneous  in  character. 
The  Function  of  the  Cerebellum. — Even  a  very  casual 
comparative  study  of  the  cerebellum  will  show  that  its  size 
and  complexity  vary  greatly  in  different  animals.  Although 


CEREBELLUM    AND    MEDULLA    OBLONGATA  343 

of  large  size  in  man  and  the  apes,  it  is  not  so  preponderant  as 
in  the  birds  and  fishes.  This  fact  leads  us  to  suspect  that 
it  is  not  related  to  intelligence,  but  rather  to  the  locomotor 
powers  of  the  animal.  In  brief,  it  appears  to  subserve  the 
co-ordination  of  muscular  movements,  and  particularly 
those  concerned  in  locomotion.  Ordinarily,  the  body  is 
in  a  condition  of  synergia,  which  implies  that  its  movements 
are  executed  with  purpose,  force,  steadiness,  and  coherence. 
Contrariwise,  an  extensive  laceration  of  the  cerebellum 
invariably  induces  a  condition  of  asynergia  or  loss  of  the 
power  properly  to  co-ordinate  muscular  movements.  This 
change  originates  in  a  loss  of  the  tonus  and  force  of  the 
skeletal  muscles,  and  an  unsteadiness  and  incoherence  in 
their  actions. 

This  view,  attributing  to  the  cerebellum  the  function  of 
an  organ  of  muscular  co-ordination,  is  based  in  a  large 
measure  upon  the  symptoms  following  the  destruction  of  this 
organ  in  certain  animals.  A  number  of  clinical  cases,  how- 
ever, have  also  been  recorded  which  show  that  the  conclusions 
drawn  from  these  experiments  on  animals  are  in  the  main 
applicable  to  man.  Inasmuch  as  the  birds  are  very  closely 
dependent  upon  a  properly  balanced  muscular  apparatus, 
it  cannot  surprise  us  to  find  that  they  are  most  profoundly 
affected  by  the  loss  of  this  organ.  Thus,  a  decerebellate 
pigeon  cannot  fly  and  cannot  even  keep  on  its  feet.  It 
loses  its  balance  with  every  move,  and  may,  in  fact,  injure 
itself  if  not  carefully  protected.  It  is  to  be  noted  especially 
that  this  loss  of  the  power  of  co-ordinated  movement  is 
not  due  to  a  paralytic  condition  of  the  skeletal  muscles, 
but  to  an  inability  on  the  part  of  the  pigeon  to  correlate  the 
contractions  of  these  organs. 

Very  similar  symptoms  are  displayed  by  the  decerebellate 
dog.  It  is  true,  however,  that  a  certain  adjustment  sets  in 
later  on,  and  that  the  aforesaid  functional  disturbances 
assume  a  milder  character  as  the  months  pass  by.  Any 
forced  movement,  however,  causes  them  to  reappear  in  all 
their  former  intensity.  Ordinarily,  any  swaying  of  the 
body  is  quickly  compensated  for  by  an  appropriate  counter- 
movement.  This  power  is  regained  to  a  certain  extent  by  the 


344  THE    NERVOUS    SYSTEM 

decerebellate  animals,  although  a  certain  awkwardness, 
unsteadiness,  and  susceptibility  to  fatigue  remain  behind 
as  permanent  results  of  this  lesion. 

By  virtue  of  its  power  to  co-ordinate  muscular  movements, 
the  cerebellum  has  always  been  regarded  as  an  important 
factor  in  the  preservation  of  the  equilibrium.  But  this 
function  cannot  be  attributed  exclusively  to  this  organ, 
because  the  sense  of  orientation  is  really  dependent  upon  a 
number  of  impressions  derived  from  such  receptors  as  the 
semicircular  canals,  the  retinae,  the  tactile  corpuscles  of 
the  skin,  and  the  muscle-spindles.  All  these  sense-organs 
unite  in  contributing  impulses  which  are  relayed  to  central 
parts  and  eventually  influence  the  musculature  through  the 
agency  of  the  cerebellum,  so  as  to  obtain  co-ordinated  and 
purposeful  muscular  responses.  Accordingly,  it  cannot 
justly  be  stated  that  this  organ  is  the  " center"  for  equili- 
bration. It  is  merely  a  link  in  the  chain  of  structures 
mediating  this  function. 

It  is  true,  however,  that  the  combined  action  of  these 
receptors  is  easily  offset.  This  fact  may  be  readily  demon- 
strated by  standing  close  to  the  edge  of  a  precipice.  Under 
ordinary  circumstances  we  make  use  of  objects  near  at  hand 
in  judging  our  position  in  space,  and  hence,  when  suddenly 
brought  into  relation  with  objects  which  are  hundreds  of 
feet  away  from  us,  our  usual  mode  of  interpreting  spacial 
relationships  no  longer  suffices  to  give  us  our  bearings. 
Vertigo,  muscular  tremors,  and  other  dangerous  symptoms 
then  develop,  which  are  easily  explained  physiologically. 
These  symptoms  may  gradually  be  mitigated  by  experience 
or  may  be  more  quickly  remedied  by  glancing  at  familiar 
objects  at  the  side  or  back  of  us. 

The  importance  of  the  eyes  in  orienting  ourselves  in- 
creases as  the  acuity  of  the  other  receptors  diminishes.  This 
point  may  well  be  illustrated  by  a  brief  consideration  of  the 
behavior  of  those  persons  whose  muscle-sense  has  been' 
partially  destroyed.  It  is  a  familiar  fact  that  the  disease  of 
locomotor  ataxia  is  characterized  by  a  degeneration  of  the 
posterior  roots  of  the  spinal  cord.  A  person  so  afflicted 
cannot  stand  erect  with  the  eyes  closed,  because  those 


CEREBELLUM    AND    MEDULLA    OBLONGATA  345 

afferent  paths  which  convey  the  sensations  from  the  muscles, 
have  been  partially  destroyed  by  this  degeneration.  He 
reels  and  falls  almost  as  soon  as  his  feet  are  brought  together 
for  the  purpose  of  testing  his  power  of  equilibration. 

The  muscle-sense  finds  its  origin  in  impulses  which  are 
produced  in  the  sensory  terminals  of  the  skeletal  muscles 
in  consequence  of  the  contraction  of  their  constituent  fibers. 
It  is  a  well  known  fact  that  we  may  close  our  eyes  and  still 
be  able  to  tell  the  position  in  space  of  any  one  of  our  parts. 
It  appears  that  these  concepts  are  the  result  of  certain 
impulses  which  arise  in  consequence  of  the  varying  degrees 
of  pressure  brought  to  bear  by  the  contracting  muscle 
fibers  upon  the  aforesaid  nerve  filaments.  Other  important 
contributing  factors  to  equilibration  &re  the  static  and 
dynamic  senses  which  are  mediated  through  the  agency  of 
the  utricle  and  semicircular  canals  of  the  internal  ear.  The 
structure  and  function  of  these  receptors  will  be  more  fully 
discussed  in  a  subsequent  chapter. 

The  Medulla  Oblongata  or  Bulb. — The  medulla  forms  the 
upper  enlarged  portion  of  the  spinal  cord  and  appears, 
therefore,  as  a  part  of  this  structure.  In  conformity  with 
its  close  anatomical  relationship  to  the  latter,  it  may  be  said 
to  be  an  organ  of  conduction  as  well  as  one  of  reflex  and  auto- 
matic action.  It  occupies  a  position  directly  in  the  path  of 
the  cerebro-spinal  fibers,  and  besides,  forms  an  important 
relay-station  upon  the  tracts  of  the  cranial  nerves.  The 
centers  contained  in  it  are  of  two  kinds:  namely,  simple, 
reflex  and  automatic.  Because  of  the  fact  that  it  embraces 
the  nuclei  of  several  cranial  nervqs,  it  becomes  the  central 
controlling  agent  of  a  number  of  simple  reflex  acts,  such  as 
are  concerned  with  the  closure  of  the  eyelids,  mastication, 
deglutition,  vomiting,  sneezing,  and  coughing. 

In  addition,  the  medulla  embraces  three  very  important 
automatic  centers:  namely,  that  regulating  the  activity  of 
the  heart,  that  controlling  the  functions  of  the  respiratory 
organs,  and  that  determining  the  caliber  of  the  bloodvessels. 
It  has  already  been  mentioned  that  the  cardiac  center  is  con- 
nected with  the  heart  by  means  of  the  vagus  nerves  and 
possibly  also  by  means  of  definite  sympathetic  fibers.  The 


346 


THE    NERVOUS    SYSTEM 


Cut  edge  of  cerebellar  peduncle 
Superior  cerebellar  peduncle 


Pineal  body 
Colliculus  superior     l 
Colliculus  inferior] 
Pulvinar  of  thalamv» 
Mesial  geniculate  body 

Jjateral  geniculate  body 

4th  nerve 
5th  nerve 


FIG.  126. — View  of  dorsal  aspect  of  upper  part  of  the  spinal  cord, 
medulla  oblongata,  pons,  fourth  ventricle,  mid-brain,  thalamus,  etc., 
dissected  in  situ.  (J.  Symington.) 


CEREBELLUM    AND    MEDULLA    OBLONGATA  347 

destruction  of  the  medulla,  however,  does  not  necessarily  stop 
the  function  of  this  organ,  because  it  is  automatically  active. 
It  then  merely  loses  those  controlling  influences  which  cor- 
relate its  activity  with  those  of  other  organs.  The  respiratory 
center  is  normally  stimulated  by  the  carbon  dioxid  of  the 
blood,  although  it  may  also  be  influenced  by  diverse  afferent 
impulses.  Inasmuch  as  it  controls  the  action  of  the  muscles 
of  respiration,  its  destruction  must  lead  to  an  almost  immedi- 
ate standstill  of  the  respiratory  mechanism,  and  hence,  termi- 
nate life  within  a  very  short  time.  The  vasomotor  center 
regulates  the  size  of  the  blood-bed.  Its  destruction  leads  to  a 
relaxation  of  the  bloodvessels,  and  fall  in  bloodpressure. 

The  Cranial  Nerves. — The  medulla  oblongata  and  parts 
above  it  give  off  twelve  pairs  of  nerves  to  each  side  of  the 
head,  which  form  an  interlocking  system  regulating  several 
independent  functions.  The  last  six  of  these  arise  from  the 
medulla  itself.  Their  course  and  function  may  be  briefly 
summarized  as  follows : 

1.  The  olfactory  nerve,  or  nerve  of  smell,  arises  in  the  sen- 
sory nerve  cells  of  the  olfactory  area  of  the  nasal  cavity.     Its 
fibers  traverse  the  pores  in  the  cribriform  plate  of  the  ethmoid 
bone,  and  by  relays  attain  the  olfactory  bulb.     The  center 
of  smell  is  situated  in  the  hippocampal  region.     Connection 
is  formed  with  the  latter  by  three  paths  which  are  known  as 
the  medial,  intermediate  and  lateral  olfactory  striae. 

2.  The  optic  nerve,  or  nerve  of  sight,  conveys  the  impulses 
from  the  retinae  to  the  thalami,  whence  they  are  transferred 
to  the  visual  center  in  the  occipital  lobes  of  the  cerebrum. 
In  man  these  fibers  effect  a  partial  crossing  in  the  optic 
chiasma.     As  has  been  stated  above,  this  crossing  permits 
those  fibers  which  are  derived  from  the  inner  halves  of  the 
retinae,   to  reach  the  opposite  half  of  the  cerebrum.     The 
outer  fibers  remain  on  the  same  side. 

3.  The  oculomotor  nerve  arises  from  a  nucleus  situated  in 
the  central  gray  matter  near  the  floor  of  aqueduct  of  Sylvius. 
This  nerve  is  motor  in  its  function  and  innervates  the  internal, 
superior  and  inferior  recti  muscles  as  well  as  the  inferior 
oblique  muscle  of  the  eyeball.     In  addition,  it  controls  the 
sphincter  muscle  of  the  iris,  as  well  as  the  ciliary  muscle. 


348 


THE   NEHVOUS    SYSTEM 


4.  The  trochlear  nerve  arises  from  a  nucleus  situated  close 
to  that  of  the  oculomotor  nerve.     It  is  a  motor  nerve,  supply- 
ing fibers  to  the  superior  oblique  muscle  of  the  eyeball. 

5.  The  trigeminal  nerve  has  two  roots,  one  motor  and  the 
other  sensory.    Near  the  apex  of  the  petrous  portion  of  the 


FIG.   127. — Trigeminus  nerve  (From  Potter,  ."Compend  of 
Human  Anatomy.") 

temporal  bone  a  large  ganglion  is  formed  which  is  known  as 
the  Gasserian  or  semilunar  ganglion.  Distally  to  this  point 
the  trigeminus  is  arranged  in  the  form  of  three  large  branches 
which  are  termed  the  ophthalmic,  superior  maxillary  and 
inferior  maxillary  nerves.  Its  ophthalmic  branch  is  princi- 
pally sensory  in  its  function,  and  supplies  the  eyeball,  lacri- 
mal  gland,  mucous  lining  of  the  eye  and  nasal  passage,  as  well 


CEREBELLUM    AND    MEDULLA    OBLONGATA  349 

as  the  integument  of  the  forehead,  nose  and  region  of  the 
eyebrows.  Its  second  division  or  superior  maxillary  branch 
is  a  sensory  nerve.  It  proceeds  forward  from  the  Gasserian 
ganglion  and  terminates  beneath  the  muse.  lev.  labii  supe- 
riores  into  a  number  of  branches  which  spread  out  upon 
the  side  of  the  nose,  lower  eyelid,  and  upper  lip.  Just  before 
this  nerve  enters  the  orbit  it  gives  off  branches  which  pass 
through  bony  canals  in  the  posterior  surface  of  the  superior 
maxillary  bone  and  eventually  reach  the  molar  teeth.  While 
traversing  the  infra-orbital  foramen,  this  nerve  also  gives  off 
branches  which  pass  to  the  upper  tricuspid,  cuspid  and  incisor 
teeth. 

The  inferior  maxillary  division  of  the  fifth  cranial  nerve 
traverses  the  foramen  ovale.  Its  motor  root  joins  the  sensory 
division  outside  the  cranium,  both  being  situated  in  the 
zygomatic  fossa.  After  their  redivision,  one  branch  is  dis- 
tributed to  all  the  muscles  of  mastication  excepting  the 
buccinator.  The  other  branch  is  chiefly  sensory  and 
divides  into  three  minor  ones :  namely,  the  auriculo-temporal, 
to  the  tissues  about  the  ear  and  the  articulation  between  the 
mandible  and  temporal  bones,  the  lingual,  to  the  tongue, 
and  the  inferior  dental  to  the  lower  teeth.  These  terminals 
are  formed  as  this  nerve  traverses  the  mandibular  canal  in 
the  body  of  the  inferior  maxillary  bone.  Just  before  enter- 
ing this  bony  channel  it  sends  some  motor  fibers  to  the 
mylohyoid  and  digastric  muscles. 

6.  The  abducens  nerve  is  a  motor  nerve  and  innervates  the 
external  rectus  muscle  of  the  eyeball.     Its  nucleus  lies  below 
the  colliculus  facialis. 

7.  The  facial  nerve  arises  in  the  tegmental  region  of  the 
pons,  and  is  chiefly  motor  in  its  function.     It  is  the  nerve  of 
expression,  because  it  controls  the  muscles  of  the  face,  those 
of  a  part  of  the  scalp,  and  those  of  the  ears.     Besides,  it 
embraces  secretomotor  and  vasomotor  fibers  for  the  sub- 
maxillary  and  sublingual  glands  which  reach  their  destination 
by  way  of  the  chorda  tympani.     It  also  innervates  the  lacri- 
mal  glands. 

8.  The  auditory  nerve  consists  of  two  groups  of  fibers, 
namely,  those  concerned  with  hearing  and  those  concerned 


350  THE    NERVOUS    SYSTEM 

with  the  sense  of  equilibrium.  The  former  pursue  a  circuitous 
course  through  the  medulla,  pons  and  lemniscus  until  they 
reach  the  psychic  area  for  audition  in  the  superior  convolution 
of  the  temporal  lobe.  Those  fibers  which  are  derived  from 
the  semicircular  canals,  are  relayed  from  the  medulla  into 
the  central  gray  matter  of  the  cerebellum. 

9.  The  glossopharyngeal  nerve  is  motor  and  sensory,  in  its 
function.     It  arises  from  the  side  of  the  medulla,  its  motor 
fibers  being  distributed  to  the  muscles  of  the  pharynx  and  its 
secreto  motor  fibers  to  the  parotid  gland.     Its  sensory  fibers 
convey  impulses  from  the  mucous  membrane  of  the  tongue, 
pharynx,  tonsils,  tympanic  cavity,  and  eustachian  tube. 

10.  The  vagus  or  pneumogastric  nerve  emerges  from  the  side 
of  the  medulla.     It  is  a  mixed  nerve.     The  preceding  discus- 
sions has  shown  that  it  is  the  principal  nerve  of  respiration, 
because  its  branches  innervate   the  larynx,   trachea,   and 
bronchi.     It  also  conveys  inhibitor  fibers  to  the  heart  and 
sensory  fibers  from  the  arch  of  the  aorta.     It  is  the  musculo- 
motor  nerve  of  'the  oesophagus,  stomach  and  intestine,  and 
sends    secretomotor  fibers  to  the   stomach,   intestine   and 
pancreas. 

11.  The  accessory  nerve  arises  from  the  medulla  and  inner- 
vates  the   sterno-cleido-mastoid   and   trapezius   muscles. 

12.  The  hypoglossal  nerve  emerges  from  the  medulla.     It 
is  a  motor  nerve  and  innervates  the  muscles  of  the  tongue, 
inclusive  of  the  geniohyoids  and  thyreohyoids. 

The  Autonomic  Nervous  System. — The  name  autonomic 
has  been  applied  to  that  part  of  the  nervous  system  which  is 
self-active,  although  primarily  controlled  by  the  higher 
centers.  Thus,  we  have  seen  that  the  heart,  the  stomach, 
intestine,  urinary  organs,  and  others  may  continue  their 
functions  even  after  they  have  been  separated  from  central 
parts,  although  they  cannot  then  be  made  to  work  in  har- 
mony with  other  structures.  This  system  of  nervous  tissue 
possesses  its  own  ganglia  and  simple  reflex  centers,  as  well 
as  intricate  networks  of  efferent  and  afferent  paths.  Its 
neurones,  however,  differ  from  those  of  the  cerebro-spinal 
system  in  their  structure  as  well  as  general  arrangement. 
Thus,  it  is  found  that  their  cell-bodies  are  usually  round, 


CEREBELLUM    AND    MEDULLA    OB  LONG  ATA  351 

while  their  axones  are  non-medullated  throughout.  Physio- 
logically, they  subserve  non-volitional  responses  and  are 
concerned  with  those  reflexes  which  are  evoked  in  the  viscera. 
Among  these  might  be  mentioned  the  movements  of  the 
heart,  oesophagus,  stomach,  intestine,  ureter,  bladder,  and 
iris.  In  this  group  also  belong  the  secretomotor,  pilomotor 
and  vasomotor  reactions. 

This  visceral  system  of  neurones  is  connected  by  several 
bridges  with  the  cerebro-spinal  system,  so  that  an  interchange 
of  impulses  may  be  effected  at  any  time.  It  is  to  be  noted, 
however,  that  this  interchange  takes  place  chiefly  in  an  efferent 
direction,  and  that  relatively  few  afferent  impulses  from  the 
viscera  actually  reach  consciousness.  One  of  these  connec- 
tions is  established  with  the  help  of  several  ganglia  situated 
in  the  paths  of  the  different  cranial  nerves,  such  as  the  fifth, 
seventh  and  tenth.  It  is  usually  designated  as  the  para- 
sympathetic  division  of  the  autonomic  system. 

Another  very  important  bridge  is  formed  at  the  level  of 
the  thoracic  segment  of  the  spinal  cord.  Certain  ganglion 
cells  which  are  located  in  the  anterior  gray  matter  of  this 
portion  of  the  cord,  send  out  fibers  which  soon  leave  the 
anterior  root  and  form  connections  with  a  number  of  ganglia 
situated  along  the  vertebral  column.  This  colony  of  ganglia 
and  their  ramification  of  fibers  constitute  the  so-called 
sympathetic  division  of  the  autonomic  system.  In  this  way 
certain  impulses  of  the  cerebro-spinal  system  are  enabled  to 
enter  the  distant  visceral  system,  because  the  aforesaid 
sympathetic  ganglia  are  closely  allied  with  the  local  nervous 
mechanisms  in  the  different  internal  organs. 

The  sympathetic  ganglia  also  embrace  a  certain  number  of 
neurones,  the  axones  of  which  pass  by  way  of  special  bridges 
into  the  cerebro-spinal  system.  These  "recurrent"  fibers 
intermingle  with  the  other  cerebro-spinal  fibers  and  jointly 
form  the  mixed  nerves  of  the  spinal  cord.  By  this  means 
the  different  sympathetic  impulses  may  also  reach  the 
smooth  muscle  fibers  of  the  skin,  the  bloodvessels  of  the 
arms  and  legs,  as  well  as  the  sweat-glands  in  all  parts  of 
the  bodv. 


PART  VI 
THE  SENSE-ORGANS 

CHAPTER  XXXVI 

THE    CUTANEOUS    SENSATIONS.     TASTE    AND 
SMELL 

General  Consideration. — We  have  regarded  the  central 
nervous  system  so  far  as  a  self-active  mechanism  which 
controls  all  the  motor  reactions  of  the  body.  In  reality, 
however,  the  motor  reactions  are  the  results  of  diverse 
afferent  impulses  which  are  constantly  poured  into  it  from  a 
number  of  sense-organs  or  receptors.  The  media  in  which 
animals  live,  are  teeming  with  diverse  manifestations  of 
energy  toward  which  they  must  orient  themselves  in  a 
very  precise  manner  in  order  not  to  endanger  their  existence. 
The  reception  and  interpretation  of  these  impressions  is  no 
less  a  duty  of  the  central  nervous  system  than  the  execution 
of  characteristic  motor  responses  for  purposes  of  adaptation. 
Thus,  the  central  nervous  system  really  serves  as  a  meeting 
place  for  diverse  afferent  and  efferent  impulses.  It  synthe- 
sizes afferent  impressions  into  motor  responses. 

Sensations  are  the  result  of  certain  processes  taking  place 
within  the  brain  in  consequence  of  impulses  derived  from  the 
distant  receptors.  Many  sensations,  however,  need  not  be 
followed  immediately  by  motor  reactions,  but  may  be  stored 
as  memory  concepts  and  called  into  play  at  any  time  later 
without  accompanying  'stimulations.  Furthermore,  sensa- 
tions are  specific,  i.e.,  they  present  different  modalities  in 
accordance  with  the  character  of  the  mechanism  producing 
them.  Thus,  a  sharp  distinction  should  invariably  be  made 
between  those  derived  from  the  retinae  of  the  eyes  and  those 

352 


THE   CUTANEOUS    SENSATIONS  353 

relayed  inward  from  the  organ  of  Corti  of  the  internal  ear, 
although  no  difference  is  discernible  in  the  character  of  the 
nerve  impulses  producing  them.  Consequently,  the  quality 
of  the  sensation  must  depend  upon  the  structural  and  func- 
tional peculiarities  of  the  center  receiving  the  impulses. 
In  analogy,  the  touch  of  an  electric  button  need  not  give  rise 
to  the  ringing  of  a  bell,  but  may  start  a  machine  or  produce 
light.  In  other  words,  the  effect  following  the  reception  of  a 
nerve  impulse  must  be  determined  by  the  structural  and 
functional  character  of  the  central  organ. 

The  quality  of  the  sensation  is  further  safe-guarded  by 
the  fact  that  the  receptor  is  specifically  adapted  to  receive 
only  one  particular  kind  of  impact.  Each  sensory  end-organ, 
so  to  speak,  can  be  excited  by  only  that  type  of  energy  in 
space  for  the  reception  of  which  it  is  peculiarly  adapted. 
Thus,  our  body  may  be  likened  to  a  house,  the  windows  of 
which  permit  the  entrance  of  light  and  sounds,  while  its  walls 
are  practically  impermeable  to  the  ordinary  physical  forces 
in  space.  Quite  similarly,  the  mass  of  our  body  is  invested 
by  a  relatively  impervious  membrane,  the  skin,  and  com- 
municates with  the  outside  only  through  particular  openings 
in  this  capsular  investment.  Furthermore,  these  openings 
are  guarded  by  specialized  nervous  mechanisms,  so  that  the 
body  may  retain  a  certain  independence  against  the  energies 
in  space  and  at  the  same  time  adapt  its  processes  to  the 
changes  in  the  medium. 

It  is  true  that  the  retina  of  the  eye  may  also  be  stimulated 
mechanically,  chemically,  and  electrically,  but  the  normal 
stimulus  is  the  light  ray.  The  former  stimuli  merely  yield 
flashes  of  light  but  no  distinct  visual  concepts.  Accord- 
ingly, it  may  be  said  that  the  adequate  stimulus  in  the  case  of 
the  retina  is  the  light  ray,  just  as  the  adequate  exciting  agent 
in  the  case  of  the  organ  of  Corti  is  the  sound  wave.  All 
other  impacts  are  inadequate,  because  they  cannot  activate 
the  sense-organs  in  a  normal  manner. 

Classification  of  the  Sense-organs. — Man  is  capable  of 
analyzing  the  manifestations  of  energy  occurring  in  space  by 
means  of  five  sense-organs:  namely,  the  retina,  organ  of 
Corti,  taste-buds,  olfactory  cells,  and  tactile  corpuscles. 

23 


354  THE    NERVOUS    SYSTEM 

The  sensations  mediated  with  the  aid  of  these  receptors  are 
classified  as  sight,  hearing,  taste,  smell  and  touch.  It  is 
true,  however,  that  the  sensations  derived  from  these  end- 
organs  may  be  divided  into  a  number  of  minor  ones,  and  that 
a  number  of  different  receptors  are  situated  in  the  internal 
structures  and  organs.  Thus,  the  skin  gives  rise  to  sensations 
or  pressure,  pain,  touch  and  temperature,  while  the  internal 
ear  contains  not  only  the  organ  of  hearing,  but  also  that  set 
aside  for  the  production  of  the  senses  of  position  and  move- 
ment. About  twenty-five  different  sensations  are  perceived 
by  us.  The  receptors  giving  rise  to  these  may  be  classified 
as  follows : 

A.  Somatic  receptors,  are  concerned  with  the  orientation 

of  the  animal  toward  its  environment, 

1.  Exteroceptors,  are  stimulated  directly  by  outside 

forces.  This  group  includes  the  retina,  organ  of 
Corti,  and  the  cutaneous  sense-organs, 

2.  Proprioceptors,   are   concerned  with  equilibrium 

and  orientation.  In  this  group  belong  the 
sensory  terminals  of  the  muscles  and  tendons, 
as  well  as  the  semicircular  canals  and  statocysts 
of  the  internal  ear, 

B.  Visceral  receptors  or  interoceptors  which  have  to  do 

with  the  sensations  arising  in  the  viscera, 

1.  General  interoceptors  which  mediate  the  sensa- 

tions of  hunger,  thirst,  nausea,  visceral  pain,  as 
well  as  those  perceived  along  the  circulatory 
and  respiratory  channels, 

2.  Special   interoceptors   which   embrace   the   end- 

organs  for  taste  and  smell. 

The  Skin  as  a  Sense-organ. — The  skin  is  exposed  to  vari- 
ous influences  which  elicit  different  kinds  of  sensations: 
namely,  those  of  pressure,  touch,  pain,  cold,  and  heat.  Sev-^ 
eral  others,  such  as  stroking  and  tickling,  are  composite  in 
their  character.  In  endeavoring  to  localize  these  sensations 
in  definite  receptors,  we  are  confronted  by  the  difficulty  that 
the  nerve-endings  of  the  integument  present  themselves  as 
a  rule  in  the  form  of  tactile  corpuscles,  i.e.,  as  bodies  which  to 


THE  CUTANEOUS  SENSATIONS 


355 


all  appearances  are  constructed  for  the  reception  of  mechani- 
cal impacts.  No  bodies  have  been  discovered  as  yet  to  which 
the  sensations  of  temperature  and  pain  could  be  singularly 
ascribed.  The  tactile  receptors,  on  the  other  hand,  usually 
exhibit  a  structure  which  leaves  no  doubt  as  to  their  function. 
They  consist  of  a  capsular  investment,  in  the  center  of  which 
lie  the  ramifications  of  the  sensory  nerve  fibers. 


Fio.  128. — Paccinian  corpuscles  from  the  peritoneum  of  a  cat.     (After 
Sala,  from  Bohm-Damdoff-Huber' s  Histology.} 

Possibly  the  most  characteristic  receiving  organ  of  this 
kind  is  the  corpuscle  of  Vater-Paccini  (Fig.  128).  It  is  oval 
in  shape;  about  2  to  4  mm.  in  length,  and  embraces  several 
concentric  rings.  Its  core  is  occupied  by  the  nerve  fiber. 
A  tactile  corpuscle  of  similar  appearance  is  that  of  Herbst 
(Fig.  129).  At  the  base  of  the  bills  of  birds  are  found  the 
corpuscles  of  Grandry  (Fig.  130),  which  consist  of  two  or  more 
hemispheral  cells,  surrounded  by  a  capsule.  The  spaces 


356  THE   NERVOUS   SYSTEM 

between  their  flattened  surfaces  are  occupied  by  the  ramifica- 
tions of  the  nerve  fiber.  While  the  sensitiveness  of  the  skin 
varies  greatly  in  its  different  regions,  it  has  been  estimated 
that  the  total  number  of  tactile  corpuscles  present  in  man, 
amounts  to  500,000.  The  skin  upon  the  back  of  the  leg 
contains  about  fifteen  to  the  square  centimeter. 


"rr-t^Bi .....  i 

—  e 


FIG.  129. — Herbst  corpuscles  of  duck,  n,  medullated  nerve-fibre; 
a,  its  axis-cylinder,  terminating  in  an  enlargement  at  end  of  core;  c, 
nuclei  of  cells  of  core;  t,  nuclei  of  cells  of  outer  tunica;  t',  inner  tunica 
X  380  diameters.  (Sobotta) 

The  structure  of  these  sense-organs  makes  it  evident  that 
their  stimulation  results  in  consequence  of  mechanical  im- 
pacts, causing  a  displacement  of  the  deeper  layers  of  the  skin. 
The  pressure  brought  to  bear  upon  their  capsular  investments 
is  communicated  in  some  inexplicable  manner  to  the  nerve 
terminals.  Nerve  impulses  arise  in  consequence  of  these 
impacts  which  then  receive  their  proper  interpretation  in 
the  corresponding  cerebral  center.  These  cutaneous  sensa- 
tions are  generally  referred  to  the  place  in  which  they  origi- 
nate, while  those  pertaining  to  sight  and  sound  are  projected 
into  space.  Thus,  a  visual  sensation  is  not  associated  as  com- 


THE  CUTANEOUS  SENSATIONS  357 

ing  from  the  retinae  of  the  eyes,  but  as  arising  in  space.  A 
similar  projection,  however,  may  at  times  be  experienced  even 
with  the  cutaneous  receptors.  Thus,  if  the  point  of  a  knife  is 
moved  across  a  rough  surface,  the  grating  sensation  does  not 
appear  to  be  derived  from  the  skin,  but  from  the  line  of  con- 
tact between  the  knife  and  the  object. 

Painful  sensations  may  be  obtained  by  the  stimulation  of 
practically  any  sense-organ.  This  is  true  of  obnoxious  odors, 
loud  sounds,  and  high  intensities  of  light.  The  ordinary 


FIG.  130. — Corpuscles  of  Grandry  from  the  duck's  tongue.  A,  com- 
pound of  three  cells,  with  two  interposed  discs,  into  which  the  axis- 
cylinder  of  the  nerve,  n,  is  observed  to  pass ;  in  B  there  is  but  one  tactile 
disc  enclosed  between  two  tactile  cells.  (Izquierdo.) 

cutaneous  sensation  of  pain,  however,  results  either  in  conse- 
quence of  the  excessive  stimulation  of  the  ordinary  receptors 
for  touch  or  in  consequence  of  the  excitation  of  specialized 
nerve-endings.  The  latter  view  is  commonly  accepted  to- 
day, although  the  structural  details  of  these  receptors  are  not 
known. 

The  integument  also  embraces  a  series  of  sensory  points 
which  yield  distinct  sensations  of  cold  or  heat.  The  cold 
spots  are  more  numerous  than  the  heat  spots,  their  rela- 
tionship being  as  13  : 1.5.  Between  these  areas  giving 
positive  reactions,  are  situated  fields  of  different  size  which 
do  not  give  distinct  sensations  of  temperature.  The  acuity 
of  this  sense  varies  greatly  in  different  regions  of  the  body. 
It  is  less  in  the  midline  of  the  body  than  in  its  lateral  regions. 

The  Sense  of  Taste. — The  end-organ  mediating  the  sense 
of  taste,  is  the  taste-bud,  so  called  because  its  cellular  ele- 
ments are  arranged  somewhat  like  the  leaves  of  a  bud.  It 
measures  80/x  in  length  and  40ju  in  breadth,  and  appears 
as  a  flask-shaped  cavity  which  is  filled  with  closely  packed 


35S 


THE    NERVOUS    SYSTEM 


elliptical  cells.  This  small  depression  in  the  mucosa  com- 
municates by  means  of  a  porous  opening  with  the  general 
cavity  of  the  mouth,  while  its  inner  pole  receives  the  terminals 
of  the  nerve  fiber  (Fig.  132). 

In  children,  the  taste-buds  are  widely  distributed  upon 
the  upper  surface  of  the  tongue,  and  the  lining  of  the  fauces, 
walls  of  the  pharynx,  and  cheeks.  In  adult  life,  on  the  other 
hand,  they  disappear  from  the  outlying  regions  and  remain 


FIG.  131.  FIG.  132. 

FIG.  131. — Diagrammatic  representation  of  circumvallate  papilla  show- 
ing the  position  of  the  taste-buds. 

FIG.  132. — Transverse  section  through  a  taste-bud.  A,  taste  pore;  B, 
spindle-shaped  cells  of  the  taste-bud;  C,  reticular  cells;  D,  nerve  fibres 
terminating  among  its  cells. 

more  closely  confined  to  the  tongue  and  fauces.  Their  wide 
distribution  explains  the  fact  that  several  nerves  are  con- 
cerned in  conveying  the  gustatory  impulses  to  the  correspond- 
ing cerebral  center.  Those  most  directly  involved  are  the 
lingual,  a  branch  of  the  inferior  maxillary  division  of  the 
fifth  cranial,  the  glossopharyngeus,  and  the  vagus. 

Upon  its  entrance  into  the  mouth,  the  food  is  subjected  to 
a  mechanical  as  well  as  chemical  reduction.  The  saliva 
plays  the  part  of  a  solvent,  because  substances  to  be  tasted 
must  be  in  the  fluid  state.  In  this  condition  they  are 'able  to 
penetrate  into  the  crevices  between  the  papillae  of  the  tongue 
and  to  form  contact  with  the  pointed  ends  of  the  gustatory 


THE   CUTANEOUS   SENSATIONS  359 

cells.  It  should  be  noted  that  the  tongue  does  not  possess  a 
perfectly  smooth  surface,  but  is  beset  with  many  minute 
papillae  which  are  either  pointed  or  rounded  at  their  apices. 
In  the  crevices  between  these  elevations  lie  the  taste-buds 
in  varying  numbers:  sometimes  one,  sometimes  several 
being  situated  in  one  of  these  pockets  (Fig.  131). 

It  is  also  to  be  noted  that  these  sense-organs  are  specific 
in  their  function,  and  give  rise  to  only  one  particular  kind 
of  taste  sensation.  Sweet  is  most  keenly  perceived  upon  the 
tip,  and  bitter  upon  the  back  of  the  tongue.  The  other 
modalities,  namely,  sour  and  salty,  are  perceived  best  upon 
the  lateral  and  antero-lateral  regions  of  this  organ. 

The  Sense  of  Smell. — The  nasal  cavity  is  lined  throughout 
with  mucous  membrane  containing  a  large  number  of  glands 
of  the  ordinary  lubricating  variety.  One  particular  area 
of  it,  however,  embraces  not  only  reticular  cells  but  also  cells 
representing  the  modified  sensory  terminals  of  the  nerve  of 
smell  or  olfactory  nerve.  These  cells  exhibit  a  spindle-like 
shape;  moreover,  their  outer  tips  are  beset  with  hair-like 
projections  which  protude  -somewhat  beyond  the  general 
surface  of  the  lining  membrane.  'The  odoriferous  particles 
contained  in  the  respiratory  air  are  brought  in  contact  with 
these  projections,  .and  in  some  way  activate  the  substance 
of  the  neighboring  olfactory  epithelium.  The  resulting  im- 
pulses are  relayed  from  here  to  the  corresponding  cerebral 
center. 

The  two  nasal  chambers  are  separated  from  one  another  by 
a  median  partition,  formed  by  the  vomer  and  adjoining 
cartilaginous  septum.  Directly  above  the  bony  palate 
separating  the  nasal  cavity  from  that  of  the  mouth,  lies  a 
relatively  large  channel  through  which  the  respiratory 
air  ebbs  back  and  forth  between  the  nostrils  and  the  posterior 
nares.  This  particular  region  of  each  nasal  chamber  is 
known  as  the  regio  respiratoria. 

The  remaining  extent  of  each  chamber  is  subdivided  into 
many  spaces  by  the  upper,  middle,  and  lower  turbinated 
or  spongy  bones  which  project  diagonally  into  the  lumen  of 
the  main  cavity.  The  surfaces  of  these  delicate  partitions 
are  covered  with  ordinary  mucous  membrane.  The  upper 


360 


THE   NERVOUS   SYSTEM 


extent  of  the  median  septum  and  opposing  surfaces  of  the 
upper  and  middle  turbinated  bones,  however,  contain  in 
addition  to  the  reticular  lining  the  aforesaid  olfactory  cells. 
The  term :  regio  olfactoria  has  been  applied  to  this  particular 
area  of  the  nasal  cavity.  It  measures  about  250  sq.  mm.  on 
each  side,  and  equals,  therefore,  the  cross-area  of  a  five-cent 
piece. 

jrtft 

'h 


FIG.  133. — Cells  of  the  olfactory  region,  a,  olfactory  cells;  6,  epithelial 
cells;  n,  central  process  prolonged  as  an  olfactory  nerve  fibril;  I,  nucleus; 
c,  knob-like  clear  termination  of  peripheral  process;  h,  olfactory  hairs. 
(After  v.  Brunn.} 

Inasmuch  as  the  air  in  the  upper  regions  of  the  nasal 
chambers  remains  relatively  stationary,  while  that  traversing 
their  lower  portions  moves  constantly  back  and  forth,  ifr 
must  be  concluded  that  the  odoriferous  particles  contained 
in  the  latter  slowly  diffuse  upward  until  they  reach  the  olfac- 
tory areas.  Accordingly,  it  will  be  seen  that  a  certain  time 
must  elapse  between  the  entrance  of  these  particles  into  the 
nasal  passages  and  the  moment  when  they  are  able  to  produce 


THE    CUTANEOUS    SENSATIONS  361 

their  characteristic  effect  upon  the  olfactory  cells.  This 
latent  period  may  be  shortened  considerably  by  the  act  of 
sniffing  which  causes  the  air  suddenly  sucked  in  through  the 
nostrils  to  be  diverted  upward  into  the  olfactory  chamber. 
It  replaces  here  the  air  just  drawn  into  the  pharynx.  The 
structural  and  functional  peculiarities  of  this  mechanism 
make  it  evident  that  a  severe  cold  which  is  characterized 
by  a  swelling  of  the  mucous  membrane,  must  give  rise  to  an 
occlusion  of  these  narrow  spaces  and  impair  the  diffusion  of 
the  odorous  particles.  It  should  also  be  remembered  that 
such  agents  as  ammonia  irritate  the  nasal  mucosa  and  excite 
the  receptors  of  the  trigeminal  nerve  rather  than  those  of  the 
olfactory  area. 


CHAPTER  XXXVII 
THE  SENSES  OF  HEARING  AND  EQUILIBRIUM 

The  External  Ear. — Sound  waves  are  vibrations  in  the 
material  medium  of  air.  They  arise  in  consequence  of 
movements  of  elastic  bodies.  Thus,  if  a  metal  plate  is  sus- 
pended and  is  struck  near  its  center  with  the  end  of  a  rod,  its 
constituent  elements  will  be  displaced  in  the  direction  of  the 
stroke.  Having  attained  an  extreme  position  in  this  direc- 
tion, they  will  then  seek  to  recover  their  original  contours 
and  places,  causing  the  plate  to  become  convex  on  the  side 
of  the  impact.  These  lateral  deviations  will  be  repeated  a 
number  of  times  until  the  entire  system  has  again  entered 
the  resting  state.,  While  the  plate  changes  its  shape,  the  air 
resting  upon  its  two  surfaces  is  alternately  put  under  a  high 
and  low  pressure,  i.e.,  the  air  lying  in  contact  with  its  convex 
surface  is  compressed,  whereas  that  upon  its  concave  surface 
is  rarefied.  In  this  way,  a  series  of  progressive  undulations 
are  produced  in1  the  air  in  harmony  with  the  elastic  qualities 
of  the  vibrator.  Some  of  these  waves  are  large  in  amplitude 
and  others  small,  and  hence,  it  may  be  said  that  they  repre- 
sent a  varying  rapidity  of  vibration  of  the  air.  The  greater 
the  number  of  these  vibrations,  the  higher  the  pitch  of  the 
sound.  The  vibratory  peculiarities  of  the  elastic  body 
determine  the  quality  of  the  sound,  while  its  intensity  or 
loudness  is  occasioned  by  the  amplitude  of  the  vibrations  of 
the  sonorous  body. 

When  a  sound  is  produced  near  at  hand,  it  seems  to 
reach  consciousness  almost  instantaneously.  We  well  know, 
however,  that  sounds  require  a  certain  period  of  time  fop 
their  transmission  through  the  air,  as  well  as  for  their  passage 
through  the  ear.  Thus,  a  distant  locomotive  is  seen  to  dis- 
charge steam  through  its  vibrator  many  seconds  before  the 
sound  produced  thereby  is  actually  perceived,  and  the 

362 


THE   SENSES   OF   HEARING   AND   EQUILIBRIUM          363 

lightning  is  seen  several  moments  before  the  thunder  is 
heard.  In  air,  the  speed  of  the  sound-waves  is  340  m.  in  a 
second,  but  other  media  conduct  it  with  an  even  greater 
rapidity.  Thus,  water  transfers  it  with  a  velocity  of  1450  m. 
in  a  second  and  wood  at  a  speed  of  13,000  m.  in  a  second. 

As  these  waves  reach  the  external  ear,  they  are  received  by 
its  funnel-shaped  pinna  and  concha,  and  are  then  reflected 
into  the  orifice  of  the  external  auditory  canal.  It  is  true, 


FIG.  134. — Diagrammatic  representation  of  the  different  parts  of  the 
ear.  1,  pinna;  2,  external  auditory  meatus;  3,  ear  drum;  4,  middle  ear 
containing  the  ossicles;  5,  Eustachian  tube;  6,  vestibule  of  the  internal 
ear;  7,  auditory  nerve;  dividing  into  two  branches,  one  of  which  innervates 
the  cochlea  and  the  other,  the  semi-circular  canals;  8,  parotid  gland. 

however,  that  the  pinna  plays  a  rather  unimportant  part 
in  the  reception  of  sounds,  because  many  animals  are  not  in 
possession  of  an  appendage  of  this  kind  and  nevertheless 
display  a  keen  sense  of  hearing.  Likewise,  a  person  whose 
pinna  has  been  removed,  does  not  reveal  any  indications  of  an 
impairment  in  his  hearing. 

The  waves  of  sound  traverse  the  external  auditory  canal 
and  finally  impinge  upon  a  membrane  which  is  placed 
obliquely  across  its  inner  lumen.  This  is  the  ear  drum  or 
tympanic  membrane.  It  consists  of  a  cartilaginous  ring, 
across  which  is  stretched  a  layer  of  fibrous  tissue  covered 


364  THE   NERVOUS   SYSTEM 

on  its  outer  side  with  delicate  skin,  and  on  its  inner  side  with 
mucous  membrane.  It  possesses  a  somewhat  oval  shape, 
and  measures  10  mm.  in  height  and  8  mm.  in  width.  Its 
area  measures  50  sq.  mm. 

The  function  of  the  ear  drum  is  to  receive  the  sound  waves 
entering  through  the  external  auditory  canal,  and  to  vibrate 
in  harmony  with  them.  Furthermore,  in  order  that  this 


FIG.  135. — Diagrammatic  representation  of  the  middle  ear  or  tympanic 
cavity.  1,  external  auditory  meatus;  2,  the  ear  drum  or  tympanic  mem- 
brane; 3,  malleus,  with  its  manubrium  resting  against  the  internal  surface 
of  the  ear  drum;  4,  incus;  5,  stapes  resting  against  the  membrane  of  the 
fenestra  ovalis;  6,  vestibule  of  the  internal  ear;  7,  fenestra  rotunda,  8, 
Eustachian  tube;  9,  saccule;  10,  central  canal  of  the  cochlea;  11,  utricle; 
12,  muse,  tensor  tympani. 

membrane  may  be  adapted  to  waves  of  different  amplitude, 
its  tenseness  is  constantly  altered  by  a  muscle  which  is 
attached  to  its  inner  surface  through  the  medium  of  the 
hammer-bone.  This  muscle  is  known  as  the  tensor  tympani. 
The  Middle  Ear. — When  we  pass  beyond  the  ear  drum 
which  closes  the  inner  orifice  of  the  external  auditory  canal, 
we  reach  the  cavity  of  the  middle  ear  or  tympanic  cavity,  in 
which  are  situated  the  ear  bones  or  ossicles.  They  are  three 
in  number:  namely,  the  hammer  bone  or  malleus,  the  anvil 
bone  or  incus,  and  the  stirrup  bone  or  stapes.  This  entire 
cavity  is  filled  with  air,  and  communicates  with  the  outside 


THE   SENSES   OF  HEARING   AND    EQUILIBRIUM          365 

only  through  a  relatively  long  and  narrow  membranous  pass- 
age, the  Eustachian  tube.  The  orifice  of  this  channel  is  situ- 
ated upon  the  posterior  wall  of  the  pharynx.  The  inner  wall 
of  the  cavity  of  the  middle  ear  is  formed  by  a  bony  septum 
which  bears  two  orifices :  namely,  the  fenestra  ovalis  and  fen- 
estra  rotunda.  Both  openings  are  closed  by  membranes. 
Inside  this  partition  lies  the  cavity  of  the  internal  ear  which  is 
filled  throughout  with  a  lymphatic  fluid  and  contains  the  most 
important  element  of  the  mechanism  of  hearing,  namely,  the 
organ  of  Corti. 

This  structural  arrangement  leads  us  to  assume  that  the 
vibrations  in  air  must  first  be  converted  into  vibrations  of 
lymph  before  the  aforesaid  receptor,  the  organ  of  Corti,  is  able 
to  receive  them.  Keeping  this  fact  clearly  in  mind,  it  may 
then  be  concluded  that. the  only  function  of  the  structures 
of  the  middle  ear  is  to  accomplish  this  transfer  of  the  sound 
waves  into  vibrations  in  lymph.  It  has  been  noted  above 
that  the  waves  transmitted  through  the  air  give  rise  to  an 
oscillation  of  the  drum  of  the  ear.  Clearly,  in  order  to  pro- 
duce similar  oscillations  in  the  lymph  of  the  internal  ear, 
the  membrane  closing  the  fenestra  ovalis  must  be  made  to 
vibrate  synchronously  with  the  ear  drum.  In  other  words, 
every  impact  upon  the  latter  must  evoke  a  corresponding 
movement  of  the  lymph  in  the  internal  ear.  The  question 
pertaining  to  the  manner  in  which  the  ear  drum  is  able  to 
produce  corresponding  displacements  of  the  membrane 
closing  the  fenestra  ovalis,  and  vibrations  in  lymph,  is  easily 
answered  by  a  brief  examination  of  Fig.  135. 

It  will  be  seen  that  the  ossicles  are  arranged  in  series,  and 
are  freely  suspended  in  the  cavity  of  the  middle  ear  by  liga- 
mentous  bands.  The  long  process  of  the  hammer  bone  lies 
in  firm  contact  with  the  inner  surface  of  the  ear  drum  and 
must,  therefore,  oscillate  in  harmony  with  this  membrane. 
It  is  moved  back  and  forth  like  the  pendulum  of  a  clock. 
Since  the  neck  of  this  bone  is  held  in  position  by  two  liga- 
ments, its  head  must  always  move  in  a  direction  opposite  to 
that  of  its  process.  Furthermore,  inasmuch  as  the  head  of 
the  malleus  is  joined  with  the  incus  by  a  broad  articulation, 
the  movements  of  the  latter  must  conform  to  those  of  the 


366  THE    NERVOUS    SYSTEM 

former.  Owing,  however,  to  a  peculiar  arrangement  of  the 
centers  of  rotation  of  these  bones,  the  stapes  which  is  at- 
tached to  one  of  the  processes  of  the  incus,  is  moved  inward 
whenever  the  process  of  the  malleus  is  forced  inward,  and 
vice  versa.  In  this  way,  every  movement  of  the  ear  drum 
is  enabled  to  leave  its  impression  upon  the  membrane  of  the 
fenestra  ovalis. 

A  proper  oscillation  of  the  ear  drum  can  only  be  effected 
if  the  pressure  upon  its  two  sides  is  equal.  This  end  is  at- 
tained by  repeated  interchanges  of  air  between  the  cavity 
of  the  middle  ear  and  that  of  the  pharynx.  The  Eustachian 
tube  to  which  reference  has  already  been  made  above,  is  the 
means  by  which  this  equalization  of  the  pressures  is  accom- 
plished. It  is  a  matter  of  common  experience  that  a  high 
atmospheric  pressure,  such  as  we  are  often  exposed  to  while 
traversing  a  tunnel,  causes  the  ear  drum  to  bulge  inward, 
thereby  diminishing  its  power  of  vibration  and  lessening  the 
acuity  of  the  sense  of  hearing.  The  peculiar  sensations  of 
pressure  then  experienced,  may  readily  be  remedied  by  the 
act  of  swallowing,  because  the  contraction  of  the  constrictors 
of  the  pharynx  opens  the  orifice  of  the  Eustachian  tube  and 
permits  air  to  enter  the  cavity  of  the  middle  ear.  As  soon 
as  the  pressures  upon  the  two  sides  of  the  ear  drum  have  been 
equalized,  it  acquires  its  former  power  of  vibration.  It  need 
scarcely  be  mentioned  that  a  rarefi  cation  of  the  outside  air 
causes  the  ear  drum  to  deviate  outward.  The  opening  of 
the  Eustachian  orifice  then  permits  ah*  to  escape  from  the 
cavity  of  the  middle  ear. 

This  brief  discussion  makes  it  evident  that  a  severe  cold 
implicating  the  mucous  lining  of  the  pharynx  and  Eustachian 
tube,  must  greatly  impair  this  interchange  of  air.  For  this 
reason,  a  certain  loss  of  the  acuity  of  hearing  is  not  an  uncom- 
mon result  of  this  affection.  Attention  should  also  be  called 
at  this  time  to  the  fact  that  septic  inflammations  of  this 
membranous  tube  may  implicate  the  mucous  lining  of  the 
tympanic  cavity,  and  eventually  give  rise  to  a  perforation  of 
the  ear  drum.  As  long  as  such  a  defect  is  not  established  in 
the  immediate  vicinity  of  the  handle  of  the  malleus,  it  need 
not  impair  the  vibratory  power  of  the  ossicles.  Furthermore, 


THE  SENSES  OF  HEARING   AND    EQUILIBRIUM  367 

the  close  proximity  of  the  cellular  spaces  of  the  mastoid  proc- 
ess of  the  temporal  bone  constitutes  a  constant  danger, 
because  any  inflammatory  process  affecting  the  middle  ear, 
may  spread  into  them  and  eventually  give  rise  to  a  perfora- 
tion into  the  cranium.  In  this  eventuality,  the  inflammation 
usually  extends  very  rapidly  along  the  coverings  of  the  brain. 
The  Internal  Ear. — In  order  to  be  able  to  understand  the 
functions  of  the  internal  ear,  it  should  be  noted  first  of  all 


FIG.  136. — Diagrammatic  view  of  the  internal  ear.  1,  tympanic  cavity; 
2,  Eustachian  tube;  3,  incus;  4,  stapes;  5,  vestibule  of  the  internal  ear 
(perilymph);  6,  utricle;  7,  central  canal  of  the  cochlea;  8,  scala  vestibuli; 
9,  saccule;  10,  endolymphatic  duct  between  saccule  and  utricle;  11, 
ampulla  of  semicircular  canal;  12,  canalis  reunions;  13,  scala  tympani; 
14,  helicotrema;  15,  fenestra  ovalis. 

that  this  structure  presents  itself  as  a  small  cavity  in  the 
petrous  portion  of  the  temporal  bone  which  is  filled  with 
lymph.  In  this  lymph  floats  a  membranous  canal  which  is 
almost  an  exact  reproduction  of  the  bony  cavity.  The 
lumen  of  this  membranous  tube  is  also  filled  with  lymph. 
Accordingly,  any  cross-section  of  the  internal  ear  presents 
first  of  all  an  outer  wall  of  bone;  then,  a  layer  of  lymph  which 
is  termed  perilymph;  next,  the  wall  of  the  membranous  tube; 


368  THE    NERVOUS    SYSTEM 

and  lastly,  the  core  of  lymph  within  the  latter  which  is  called 
endolymph. 

It  should  also  be  noted  that  the  internal  ear  contains  not 
only  the  receptor  for  the  sound  waves  or  organ  of  Corti,  but 
also  one  concerned  with  the  production  of  the  sense  of  equili- 
brium and  situated  in  the  ampullae  of  the  labyrinth.  Thus, 
the  internal  ear  may  be  divided  into  two  distinct  parts: 
namely  the  cochlea,  in  which  is  located  the  organ  of  Corti 
mediating  the  sensations  of  hearing,  and  the  saccule,  utricle 
and  semicircular  canals,  in  which  those  sensations  arise 
which  enable  us  to  orient  ourselves  in  space. 

The  cochlea  is  a  snail-like  structure  situated  anteriorly  to 
the  vestibule  (fen.  ovalis)  of  the  internal  ear.  It  measures  9 
mm.  across  its  base  and  5  mm.  from  base  to  apex  This  bony 
tube  is  wound  upon  itself  two  and  one-half  times,  and  con- 
tains a  membranous  tube  which  is  closely  adherent  to  its 
wall  in  several  places,  so  that  its  lumen  becomes  subdivided 
into  three  separate  passages.  This  arrangement  may  best 
be  illustrated  with  the  aid  of  Fig.  137,  representing  a  cross- 
section  of  the  canal  of  the  cochlea  at  practically  any  level. 
It  will  be  seen  that  its  lumen  is  partially  divided  into  two 
by  a  bony  plate  which  projects  almost  horizontally  outward 
from  the  central  core  of  bone  around  which  the  canal  is 
wound.  This  division  is  made  complete  by  a  membranous 
septum  which  is  fastened,  on  the  one  hand,  to  the  tip  of  this 
bony  plate  and,  on  the  other,  to  the  outer  wall  of  the  canal. 
It  is  called  the  basilar  membrane.  Above  this  septum  lies  the 
scala  vestibuli,  so-called  because  it  leads  from  the  region  of 
the  fenestra  ovalis  or  vestibule  of  the  internal  ear  into  the 
tip  of  the  cochlear  canal,  a  distance  of  about  33  mm.  Below 
this  septum  lies  the  scala  tympani  which  is  connected  with 
the  former  in  the  tip  of  the  canal  and  terminates  blindly  at 
the  fenestra  rotunda. 

It  is  to  be  noted  especially  that  the  scala  vestibuli  and  scala 
tympani  are  perilymph  canals.  The  endolymph  canal  is 
represented  by  the  central  canal,  the  floor  of  which  cor- 
responds to  the  aforesaid  membranous  septum,  while  its 
roof  is  formed  by  a  delicate  membrane  which  stretches  ob- 
liquely through  the  vestibular  scala.  The  latter  is  termed 


THE    SENSES    OF    HEARING   AND    EQUILIBRIUM 


369 


the  membrane  of  Reissner.  The  lining  of  the  upper  surface 
of  the  basilar  membrane  is  modified  to  give  lodgment  to  the 
terminals  of  the  auditory  nerve.  The  cells  assume  here  a 
peculiar  shape  and  acquire  hair-like  processes  upon  their 


FIG.  137. — Diagram  of  a  transverse  section  of  the  cochlea.  Sc.  V, 
scala  vestibuli;  Sc.  T,  scala  tympani;  C.  Chi,  canalis  cochlearis;  Lam.  sp., 
lamina  spiralis;  Gg.  sp,  ganglion  spirale;  n.  and,  auditory  nerve;  m.R, 
membrane  of  Reissner;  Sv.tr.  •  stria  vascularis;  Lg.sp,  ligamentum  spirale; 
t.l,  lymphatic  epithelioid  lining  of  basilar  membrane  on  the  tympanic 
side;  m.  b,  basilar  membrane;  Org.  C,  organ  of  Corti;  L.t,  labium  tym- 
panicum;  Ib,  limbus;  L.v,  labium  vestibulare;  m.t,  tectorial  membrane. 
(After  Foster.) 


outer  surfaces  which  project  free  into  the  lymph  of  the 
central  canal.  These  hair  cells,  together  with  a  number  of 
reticular  cells,  constitute  the  organ  of  Corti  which  is  respon- 
sible for  the  reception  of  the  sound  waves. 

24 


370  THE    SENSE-ORGANS 

The  manner  of  activation  of  this  receptor  may  be  briefly 
outlined  as  follows :  The  impacts  of  the  stapes  upon  the  mem- 
brane of  the  fenestra  ovalis,  executed  in  harmony  with  the 
vibrations  of  the  ear  drum,  give  rise  to  vibrations  in  the  peri- 
lymph  of  the  internal  ear.  The  latter  ascend  through  the 
scala  vestibuli  and  descend  through  the  scala  tympani. 
This  implies  that  the  membrane  closing  the  fenestra  ovalis 
must  vibrate  synchronously  with  the  membrane  of  the  fenes- 
tra rotunda,  but  in  opposite  directions  to  one  another.  In 
this  way,  certain  interchanges  of  pressure  are  effected  which 
permit  the  membrane  of  the  fenestra  ovalis  to  oscillate  with 
the  greatest  possible  freedom.  Since  the  endolymph  of 
the  central  canal  in  which  the  organ  of  Corti  is  situated,  is 
separated  from  the  perilymph  in  the  vestibular  scala  by  only 
a  very  thin  membrane,  the  vibrations  of  the  latter  are  easily 
communicated  to  the  former.  It  appears,  that  the  vibra- 
tions in  the  endolymph  then  evoke  movements  of  the  hair- 
like  processes  upon  the  cells  of  Deiters  which  in  some  way 
activate  the  terminals  of  the  auditory  nerve. 

It  is  obvious,  therefore,  that  the  hair-cells  of  the  organ  of 
Corti  play  the  part  of  resonators.  As  such  they  are  able  to 
analyze  the  different  vibrations  in  lymph,  corresponding  in 
turn  to  the  varying  amplitudes  of  the  sound  waves.  The 
action  of  these  hair-cells  is  specific,  i.e.,  any  given  cell  appears 
to  be  able  to  receive  only  one  particular  kind  of  vibration. 
The  general  phenomenon  here  involved  is  familiar  to  practi- 
cally everybody.  Thus,  if  a  sound  is  produced  in  the  vicin- 
ity of  a  string-instrument,  only  that  string  will  be  set  into 
sympathetic  vibration  by  it  which  is  structurally  adapted 
to  receive  it.  The  total  number  of  hair-cells  is  usually  given 
as  16,000,  a  number  sufficient  to  allow  us  to  analyze  musical 
sounds  of  between  16  and  30,000  vibrations  per  second.  In 
fact,  the  trained  musical  ear  is  able  to  recognize  even  sounds 
of  50,000  vibrations  per  second. 

The  Semi- circular  Canals. — In  many  of  the  lower  animals 
the  organ  of  equilibrium  consists  merely  of  a  saccular  inden- 
tation of  the  integument  which  is  lined  with  cuboidal  cells 
possessing  hair-like  projections.  The  ends  of  these  filaments 
are  often  weighted  with  minute  calcareous  concretions  which 


THE   SENSES   OF   HEARING    AND   EQUILIBRIUM          371 


are  known  as  otolyths.  A  peculiar  semi-liquid  material 
occupies  the  remaining  portion  of  this  cavity.  This  entire 
structure  is  termed  a  statocyst,  or  otolithic  cavity,  and  its  func- 
tion is  to  mediate  the  static"  sense  or  sense  of  position.  The 
animals,  possessing  end-organs  of  this  character,  move 
principally  along  straight  lines  and  are  not  subjected  to 
rotary  movements;  hence,  a  simple  receptor  of  this  kind  is 
really  all  they  require  in  order  to  be 
able  to  ascertain  their  position  in 
space.  It  is  evident  that  any  change 
in  the  position  of  the  head  of  the 
animal  must  cause  the  hair-like  pro- 
jections of  the  lining  cells  of  the 
statocysts  to  be  deviated  in  conse- 
quence of  the  changes  in  the  pres- 
sure of  the  material  overlying  them. 
These  deviations  activate  the  sen- 
sory fibers  connected  with  the  basal 
portions  of  these  cells.  Sensory 
epithelium  of  this  kind  is  also  pres- 
ent in  the  human  ear,  but  only  in 
the  utricular  portion  of  the  mem- 
branous canal. 

The  principal  receptor  mediating 
sensations  of  position  lies  in  the  ampullce  of  the  semicircular 
canals.  If  we  pass  backward  from  the  vicinity  of  the  fen- 
estra  ovalis,  we  find  that  the  bone  is  hollowed  out  in  the 
form  of  three  narrow  canals,  each  describing  a  half-circle. 
Inside  these  lie  membranous  tubes  of  the  same  general  shape 
which  begin  and  terminate  at  the  utricle,  a  spacious  enlarge- 
ment of  the  endolymph  sac.  Within  a  short  distance  of  the 
utricle,  each  semicircular  canal  presents  a  bulbular  enlarge- 
ment, which  is  termed  the  ampulla.  The  lining  of  this  par- 
ticular segment  of  the  canal  is  raised  in  the  form  of  a 
transverse  ridge,  and  is  made  up  of  high  cells  bearing  hair- 
like  projections  which  float  free  in  the  endolymph.  This 
structure  is  known  as  the  crista  acustica.  It  is  supplied  by 
fibers  from  the  vestibular  branch  of  the  auditory  nerve. ' 

It  should  be  noted  next  that  these  canals  are  arranged  in 


FIG.  138. — The  otolithic 
cavity  showing  the  lining 
cells  with  their  hair-like 
prolongations  and  the 
otoliths. 


372 


THE    SENSE-ORGANS 


CEREBRUM 


SEMICIRCULAR 
MEDULLA  OBLOK/«AT 


FIG.  139.  —  The  semicircular  canals  in  the  pigeon. 


FIG    140.— Figure  showing  the  position  of  the  three  semicircular  canals 
in  the  skull  of  the  pigeon.     (Ewald.) 


THE    SENSES    OF   HEARING   AND    EQUILIBRIUM  373 

such  a  way  that  they  cover  three  distinct  planes  in  space, 
situated  approximately  at  right  angles  to  one  another.  One 
canal  is  placed  horizontally,  while  the  other  two  are  directed 
vertically  when  the  head  is  held  erect.  The  two  vertical 
canals  deviate  from  the  mesial  plane  at  an  angle  of  45°; 
hence,  if  the  positions  of  the  right  and  left  canals  are  com- 
pared with  one  another,  it  will  be  found  that  the  left  anterior 
covers  the  same  plane  as  the  right  posterior.  Quite  simi- 
larly, the  right  anterior  is  supplemented  by  the  left  posterior. 
Movements  in  any  direction  not  in  line  with  the  planes  of 
these  canals  activate  two  canals  in  an  unequal  measure. 


FIG.  141. — Diagrammatic  representation  of  the  structure  of  the  ampulla 
of  a  fish.  The  columnar  cells  of  the  crista  acustica  (c)  are  beset  with  hair- 
like  prolongations  which  float  free  in  the  endolymph.  N,  nerve  fibers 
leading  away  from  ampulla. 

The  fact  that  these  structures  are  intimately  concerned 
with  equilibration  has  been  amply  proven,  because  their 
destruction  gives  rise  to  disorders  in  the  power  of  retaining 
our  position.  The  birds  are  especially  adapted  for  experi- 
ments of  this  kind,  because  their  labyrinth  is  very  accessible 
to  operative  interference.  Thus,  a  pigeon  whose  canals  have 
been  extirpated  on  one  side,  is  quite  unable  to  maintain  its 
position.  If  it  is  made  to  move,  it  sways  and  tumbles  toward 
the  side  of  the  injury.  Its  head  is  tilted  toward  one  side  and 
may  even  be  inverted.  A  certain  adjustment,  however, 
takes  place  in  the  course  of  time,  so  that  an  animal  of  this  kind 
will  not  show  such  serious  disturbances  when  allowed  to 
remain  relatively  quiescent. 


374  THE   SENSE-ORGANS 

The  manner  of  activation  of  this  receptor  may  readily  be 
deduced  from  its  histological  structure.  Whenever  the  head 
is  moved  in  a  particular  direction,  the  lymph  within  the 
membranous  canal  is  also  moved,  thereby  causing  the  hair 
processes  to  be  deviated  from  their  position  of  rest.  Al- 
though gravity  cannot  be  excluded  altogether,  it  is  evident 
that  this  receptor  is  peculiarly  adapted  to  movements  and 
mediates,  therefore,  the  dynamic  sense.  In  man,  the  purely 
static  sensations  are  believed  to  be  received  by  the  sensory 
epithelium  of  the  utricle  and  saccule. 


CHAPTER  XXXVIII 
THE  SENSE  OF  SIGHT 

The  Nature  of  the  Stimulation  by  Light. — The  sources  of 
light  are  either  natural  or  artificial.  In  the  former  group 
belong  the  sun,  stars,  comets,  meteors,  and  phosphorescent 
bodies,  and  in  the  latter  the  combustions  of  gas,  oil,  wood, 
coal,  and  other  substances.  Regarding  the  cause  of  light, 
two  theories  have  been  propounded  which  are  characterized 
respectively  as  the  emission  or  corpuscular  and  the  undulatory. 
The  former  regards  light  as  minute  particles  which  are 
discharged  by  the  luminous  body  in  straight  lines,  while  the 
latter  assumes  that  combustions  give  rise  to  vibrations  in 
ether,  the  attenuated  medium  filling  space.  These  vibrations 
are  propagated  with  an  almost  inconceivably  rapid  rate, 
usually  estimated  at  about  190,000  miles  in  a  second. 

In  their  passage  through  space  these  vibrations  are  brought 
in  contact  with  different  bodies,  which  are  classified  as: 
transparent,  when  they  permit  the  passage  of  white  light 
and  its  spectral  components  so  that  the  object  may  be  seen 
in  its  colors:  translucent,  when  only  a  certain  number  of  rays 
are  able  to  pass  outlining  the  object  as  a  shadow,  and  opaque, 
when  the  rays  cannot  get  beyond  them.  Furthermore,  an 
opaque  body  may  cause  a  certain  number  of  the  rays  to  be 
absorbed  and  another  to  be  reverberated.  In  the  latter 
case,  the  light  changes  its  direction,  although  continuing  in 
the  same  medium.  The  term  of  reflection  is  applied  to  this 
phenomenon.  If  a  ray  of  light  is  made  to  pass  from  one 
medium  into  another  in  an  oblique  direction,  it  is  deviated 
from  its  course.  This  phenomenon  is  characterized  as 
refraction. 

Light  is  the  most  potent  stimulant  of  living  matter.  The 
lowest  forms  require  it  in  optimum  intensities.  Any  greater 
or  lesser  amount  acts  destructively  upon  them.  Thus  we 

375 


376  THE    SENSE-ORGANS 

find  that  such  organisms  as  the  amoeba  behave  toward  it  in 
a  particular  manner,  being  attracted  to  it  when  its  intensity 
is  low,  and  repelled  when  its  intensity  is  high.  In  other 
words,  these  organisms  orient  themselves  toward  light  in  a 
positive  as  well  as  negative  manner,-  but  in  all  these  instances 
the  stimulus  is  brought  to  bear  upon  their  substance  in  a 
direct  way  and  in  all  probability  not  through  special  receiving 
organs.  Furthermore,  the  effects  of  these  impacts  are 
limited  to  changes  which  give  rise  solely  to  particular  move- 
ments. The  term  of  heliotropism  or  phototaxis  is  applied  to 
the  power  of  .simple  organisms  to  orient  themselves  in  accor- 
dance with  the  intensity  and  direction  of  the  light  rays. 

Somewhat  higher  in  the  scale  of  the  animal  kingdom,  we 
note  the  development  of  the  so-called  eye-spots,  which  first 
appear  in  the  form  of  globules  of  a  peculiar  substance  ex- 
tremely sensitive  to  light.  But  even  this  receptor  substance 
does  not  seem  to  mediate  anything  more  complex  than  the 
estimation  of  varying  degrees  of  light :  i.e.  distinct  imprints  of 
objects  cannot  be  obtained  with  the  aid  of  this  material.  In 
the  higher  invertebrates,  the  sensitive  epithelium  is  spread 
out  in  the  form  of  a  hemispherical  layer  and  is  invested  by 
structures,  the  principal  purpose  of  which  is  to  bring  the 
rays  of  light  to  a  concise  intersecting  point  upon  it.  Thus, 
the  eye  of  the  insects  presents  numerous  funnel-shaped  tubes 
through  which  the  light  is  refracted  by  means  of  delicate 
lenses  of  chitin.  This  type  of  eye,  however,  is  soon  aban- 
doned and  gives  way  to  one  possessing  a  single  system  of 
curved  refracting  media.  It  thus  acquires  a  striking  simi- 
larity to  a  camera  obscura,  the  box  of  which  is  represented  by 
the  wall  of  the  eyeball,  its  refracting  medium  by  the  cornea 
and  lens,  and  its  sensitive  screen  by  the  retina. 

Naturally,  the  most  important  constituent  of  the  eye  is  its 
sensory  epithelium,  because  it  permits  the  transfer  of  the 
ethereal  vibrations  into  sensations  of  light  which  are  then 
relayed  to  the  corresponding  center  in  the  cerebrum  to  be 
associated.  Thus,  the  cornea,  lens,  and  humours  of  the  eye 
should  be  regarded  merely  as  adjuncts,  by  means  of  which  the 
rays  of  light  are  centralized  upon  this  receptor  in  the  most 
efficient  manner.  In  this  connection,  it  should  also  be  men- 


THE    SENSE    OF    SIGHT 


377 


tioned  that  this  receptor  may  be  excited  by  mechanical  and 
electrical  stimuli,  but  these  impacts  give  rise  solely  to  sensa- 
tions of  light  which  are  commonly  known  as  phosphenes,  and 
not  to  distinct  reproductions  of  outside  conditions.  Associa- 
tive imprints  are  formed  only  in  consequence  of  stimulations 
by  light.  Accordingly,  the  only  adequate  stimuli  in  the  case 
of  the  retina  are  the  ethereal  vibrations. 

The  General  Structure  of  the  Eye. — The  organ  of  sight  is 
arranged  in  a  bilateral  manner.     It  consists  of  two  globular 


FIG.  142. — Diagram  of  a  horizontal  section  through  the  human  eye, 
C,  cornea;  A,  anterior  cavity;  P,  posterior  cavity;  L,  lens;  J,  iris;  T, 
conjunctival  sac;  CL,  ciliary  ligament;  CB,  ciliary  body;  CM,  ciliary 
muscle;  OS,  ora  serrata;  CS,  canal  of  Schlemm;  R,  retina;  Ch,  choroid; 
S,  sclera;  ON,  optic  nerve;  A,  retinal  artery;  B,  blind  spot;  Y,  yellow  spot; 
OA,  optical  axis;  VA,  visual  axis;  H,  hyaloid  canal. 

bodies,  the  eyeballs,  which  are  situated  in  the  orbital  cavities 
of  the  skull.  Each  eye  ball  is  invested  by  a  capsule,  the  sur- 
faces of  which  are  moistened  with  a  lymphatic  fluid  to  facili- 
tate its  movements.  In  front  it  is  protected  by  the  eyelids 


378  THE   SENSE-ORGANS 

which  appear  primarily  as  reflections  of  the  skin,  covering 
almost  one-half  of  its  entire  outer  surface.  Those  regions  of 
the  lids  which  lie  in  relation  with  the  eyeball,  are  lined  with 
mucous  membrane  and  are  constantly  moistened  with  a 
watery  fluid,  secreted  by  the  lacrimal  gland.  This  struc- 
ture is  placed  in  a  depression  in  the  horizontal  plate  of  the 
frontal  bone  above  the  outer  corner  of  the  cleft  between 
the  lids. 

After  this  secretory  product  has  been  discharged  into  the 
conjunctival  sac,  it  is  spread  by  capillarity  across  the  surface 
of  the  eyeball,  thereby  tending  to  keep  the  latter  from  drying 
and  to  remove  from  it  all  particles  of  dust  that  may  have 
accumulated  upon  it.  The  tears  finally  escape  from  the 
conjunctival  sac  through  two  small  openings  in  the  inner 
angles  of  the  lids.  They  then  enter  the  nasal  cavity  by  way 
of  the  nasal  duct,  the  orifice  of  which  is  situated  in  the  upper 
portion  of  this  recess.  Thus,  lacrimation  must  always  be 
followed  by  the  escape  of  an  extra  amount  of  fluid  from  this 
passage.  Under  ordinary  conditions,  the  tears  do  not  flow 
across  the  edges  of  the  lids,  because  the  inner  margins  of  the 
latter  are  equipped  with  a  large  number  of  modified  sebaceous 
glands,  secreting  an  oily  liquid.  These  are  the  Meibomian 
glands.  Their  oily  product  also  serves  as  a  lubricant  for  the 
hairs  with  which  the  outer  margins  of  the  lids  are  beset  as  a 
means  of  protecting  the  eyeball  against  dust. 

The  waU  of  the  eyeball  is  composed  of  three  layers  of  tissue : 
namely,  a  dense  outer  covering  or  sclera,  a  delicate  vascular 
layer  or  choroid,  and  an  inner  coat  of  sensory  epithelium  or 
retina.  The  strength  and  resistance  of  the  eyeball  are  due, 
of  course,  to  its  connective  tissue  envelope  in  the  form  of  the 
sclera,  while  its  nutrition  is  effected  chiefly  by  the  vessels  of 
the  choroid.  Within  this  rounded  capsule  the  retina  is 
expanded  in  the  form  of  a  hemispherical  funnel,  its  concave 
surface  being  directed  toward  the  anterior  pole  of  the  eyeball, 
while  its  convex  posterior  surface  lies  everywhere  in  intimate 
contact  with  the  choroid.  The  nerve  fibers  of  the  retina 
strive  radially  toward  a  common  center  and  leave  the  eyeball 
near  its  posterior  pole  in  the  form  of  the  optic  nerve. 

The  retina,  as  well  as  the  choroid,  covers  the  sclera  for 


THE   SENSE   OF    SIGHT  379 

only  about  three-fourths  of  its  extent.  Furthermore,  at 
about  the  line  of  junction  between  the  anterior  one-fourth  and 
posterior  three-fourths  of  the  eyeball  the  sclera  becomes 
transparent.  This  permeable  segment  of  this  fibrous  capsule 
is  named  the  cornea.  It  is  more  convex  than  its  opaque  pos- 
terior part,  so  that  the  eyeball  as  a  whole  really  possesses  the 
shape  of  two  telescoped  spheroids,  its  corneal  vestibule 
being  cut  out  of  a  smaller,  and  hence,  more  convex  sphere. 

If  we  now  observe  the  eyeball  in  longitudinal  section,  it 
will  be  seen  that  its  external  characteristics  correspond  very 
closely  to  its  internal  structure.  It  is  to  be  noted  first  of  all 
that  its  cavity  is  subdivided  into  an  anterior  and  a  posterior 
chamber  by  a  vertical  partition  consisting  of  the  ciliary  body, 
lens  and  iris.  Both  chambers  are  filled  with  fluid,  that  in  the 
anterior  compartment  exhibiting  the  consistency  of  lymph, 
and  that  in  the  posterior  cavity  the  consistency  of  a  delicate 
jelly.  The  former  is  designated  as  aqueous  humour,  and  the 
latter,  as  vitreous  humour.  It  is  to  be  noted  especially  that 
these  fluids  are  held  under  a  certain  pressure,  tending  to  keep 
the  different  membranes  and  partitions  of  the  eyeball  fully 
expanded,  so  as  to  obtain  the  best  possible  refraction  of  the 
rays  of  light 

The  External  Muscles  of  the  Eyeball. — In  order  to  be  able 
to  bring  the  rays  of  light  emitted  by  objects  in  space  to  a  pre- 
cise focal  point  upon  the  retinae,  it  is  essential  that  the  eye- 
balls be  moved  in  the  direction  of  the  object  by  local  muscular 
action.  Any  additional  deviation  that  may  be  needed  to  ac- 
complish this  end,  is  attained  by  moving  the  head  and  body 
as  a  whole.  The  muscles  of  the  eyeball  originate  upon  the 
walls  of  the  orbital  cavity  in  the  vicinity  of  the  optic  foramen, 
and  pass  forward  to  be  inserted  upon  the  eyeball  back  of  the 
cornea.  They  are  six  in  number:  namely,  four  straight  ones 
and  two  oblique  ones.  The  first  are  designated  respectively 
as  the  superior,  inferior,  external  and  internal  recti  muscles, 
and  the  latter,  as  the  superior  and  inferior  oblique  muscles. 

Inasmuch  as  the  eyeball  is  moved  in  a  capsule  around  its 
horizontal,  vertical  and  oblique  axes,  it  is  evident  that  the 
contraction  of  the  superior  rectus  turns  the  cornea  up- 
ward, while  that  of  the  inferior  rectus  turns  it  downward. 


380  THE   SENSE-ORGANS 

Quite  similarly,  the  external  rectus  deviates  the  long  axis  of 
the  eye  outward.  The  reverse  position  is  given  to  it  by  the 
contraction  of  the  internal  rectus. 

The  action  of  the  upper  oblique  muscle  is  easily  understood 
if  it  is  remembered  that  its  tendon  traverses  a  pully-like  loop 
before  it  is  actually  inserted  upon  the  outer  aspect  of  the 
eyeball  somewhat  behind  its  center.  Owing  to  the  peculiar 
course  followed  by  this  muscle,  its  contraction  must  turn  the 
cornea  downward  and  inward.  The  lower  oblique  muscle 
arises  from  the  anterior  margin  of  the  orbital  cavity  exter- 
nally to  the  orifice  of  the  nasal  duct,  and  passes  backward, 
outward,  and  upward  to  be  inserted  into  the  outer  sclera. 
It  turns  the  cornea  upward  and  inward. 

It  is  usually  believed,  however,  that  the  oblique  muscles 
do  not  act  singly  but  only  in  conjunction  with  the  recti 
muscles.  Furthermore,  the  innervation  of  the  muscles  on 
the  two  sides  is  adjusted  in  such  a  way  that  the  object  is 
focalized  upon  Jiarmoneous  areas  of  the  retinae.  Thus, 
when  gazing  at  a  near  point,  the  eyes  are  converged  equally 
by  the  internal  recti  muscles,  so  that  the  rays  of  light  emitted 
by  this  object  fall  upon  corresponding  points  of  the  retinae. 


CHAPTER  XXXIX 

THE  COURSE  OF  THE  RAYS  OF  LIGHT  THROUGH 
THE  EYE 

The  Cornea  and  Aqueous  Humour. — Any  luminous  point 
in  space  emits  rays  of  light  radially  in  all  directions.  A  certain 
number  of  these  always  pursue  a  course  parallel  to  the  long 
axis  of  the  eyeball,  provided  the  object  is  situated  at  a  dis- 
tance of  at  least  5  to  6  m.  As  these  rays  impinge  upon  the 
cornea,  they  are  refracted,  because  this  medium  possesses  a 
much  greater  density  than  the  air.  Similar  deviations  of  the 
rays  are  effected  at  other  lines  of  contact  between  the  different 
media  of  the  eyeball,  the  tendency  always  being  to  converge 
them  into  a  precise  intersecting  point  upon  the  retina.  As 
the  different  luminous  points  of  an  object  are  focalized  in  this 
way  upon  this  screen,  they  attain  definite  values  as  lights  and 
shadows  and  even  as  colors  which  are  then  properly  associated 
in  the  corresponding  center  of  the  cerebrum. 

The  cornea,  therefore,  plays  the  part  of  a  converging  plane 
which  directs  the  rays  of  light  into  the  eye.  Thus,  many  of 
those  which  strike  its  surface  slantingly  and  would  therefore 
be  lost,  are  deviated  sufficiently  from  their  course  to  allow 
them  to  reach  the  interior  of  the  eye.  Having  traversed  the 
aqueous  humour,  the  rays  are  rendered  strongly  convergent 
by  the  crystalline  lens. 

The  Iris. — It  should  be  noted,  however,  that  only  those 
rays  actually  reach  the  lens  which  traverse  the  central  area 
of  the  cornea,  because  only  these  are  able  eventually  to 
engage  in  the  pupillar  orifice  of  the  iris.  A  glance  at  Fig.  142 
will  show  that  the  lens  is  protected  in  front  by  a  thin  membran- 
ous diaphragm  which  possesses  the  same  purpose  as  the  stop 
of  a  photographic  camera.  This  statement  implies  that  the 
opening  between  its  margins  may  be  altered  in  size,  thereby 
permitting  varying  numbers  of  light  rays  to  enter  the  interior 
of  the  eyeball.  Thus,  only  the  centralmost  segment  of  the 

381 


382  THE    SENSE-ORGANS 

crystalline  lens  is  actually  exposed  to  the  light,  while  its 
outer  area  is  covered  by  the  iris.  This  is  of  greatest 
importance,  because  it  is  a  well  established  fact  that  the 
central  portion  of  a  convex  lens  refracts  most  perfectly. 
Contrariwise,  its  peripheral  segments  are  prone  to  converge 
the  rays  in  different  directions,  so  that  caustics  are  formed. 
It  may  be  said,  therefore,  that  the  iris  possesses  two  functions : 
namely,  that  of  varying  the  size  of  the  bundle  of  light  which  is 
permitted  to  stimulate  the  retina,  and  that  of  directing  the 
rays  through  the  most  perfect  central  area  of  the  lens. 

These  changes  may  easily  be  observed  in  any  person  when 
he  is  requested  to  gaze  alternately  at  a  bright  light  and  a 
dark  wall.  In  the  former  instance,  the  pupil  becomes 
smaller,  so  as  to  protect  the  retina  against  the  entrance  of 
an  excessive  number  of  light  rays.  It  will  be  remembered 
that  the  constriction  of  the  pupil  is  caused  by  the  contraction 
of  a  group  of  smooth  muscle  cells  which  traverse  the  sub- 
stance of  the  iris  in  a  circular  direction,  while  the  enlargement 
or  dilatation  of  this  orifice  is  accomplished  by  the  contraction 
of  its  radial  muscle  fibers  and  consequent  retraction  of  the 
margins  of  the  iris. 

Naturally,  the  pupil  of  the  eye  must  appear  dark  to  the 
observer,  because  the  back  of  the  eye  is  not  luminous,  and 
hence,  cannot  emit  rays  of  light  which  are  projected  outward 
through  this  orifice.  It  can  be  rendered  luminous,  how- 
ever, by  reflecting  light  into  the  eye  by  means  of  a  mirror. 
Having  in  this  way  lighted  up  the  retina  or  fundus  of  the 
eye,  a  large  number  of  luminous  points  are  formed  which 
send  rays  outward  into  space.  These  are  then  focalized  in 
the  observer's  eye.  Furthermore,  the  iris  itself  is  practi- 
cally impermeable  to  the  rays  of  light,  because  it  contains  a 
certain  amount  of  pigment,  which  lends  color  to  the  eye  as 
a  whole.  The  heavier  this  deposit,  the  darker  its  color. 
Consequently,  the  blue  color  of  the  iris  signifies  that  it 
embraces  a  smaller  amount  of  pigment  than  one  giving  the 
sensation  of  brown.  The  eyes  of  albinos  are  deficient  in 
pigment  and,  hence,  these  persons  endeavor  to  shield  the 
retinae  against  an  excessive  stimulation  by  the  light  rays 
by  partially  closing  the  eyelids.  Furthermore,  an  eye  of  this 


THE    RAYS    OF   LIGHT   THROUGH   THE    EYE  383 

character  appears  pink,  because  it  allows  a  large  number  of 
reflected  rays  to  pass  outward  into  space. 

It  need  scarcely  be  emphasized  that  the  constriction  and 
dilatation  of  the  pupil  cannot  be  effected  volitionally. 
Both  are  reflex  acts,  instituted  by  the  excitation  of  the  light 
rays.  Those  changes  which  result  in  consequence  of  varia- 
tions in  the  intensity  of  the  rays,  constitute  the  so-called 
light-reflex.  Very  similar  alterations  are  noted  when  the 
eye  is  alternately  fixed  for  far  and  near  objects.  In  the  latter 
instance,  the  pupil  becomes  smaller  in  order  to  restrict  the 
size  of  the  bundle  of  light.  Contrariwise,  it  is  a  requirement 
of  distinct  far  vision  that  every  available  ray  be  permitted 
to  enter  the  eyeball.  Consequently,  the  pupil  is  dilated  at 
this  time.  These  changes  constitute  the  so-called  accommo- 
dation-reflex. 

The  Lens  and  Ciliary  Body. — While  the  contents  of  the 
eyeball  are  adjusted  in  a  manner  to  act  as  a  biconvex  lens,  the 
principal  refraction  takes  place  at  the  crystalline  lens  which 
is  suspended  directly  in  the  visual  path  by  means  of  ligaments 
attached  to  the  ciliary  body.  This  structure  is  formed  by  a 
duplication  of  the  choroid  coat  which,  so  to  speak,  is  fastened 
to  the  sclerotic  capsule  of  the  eyeball  at  about  the  junction 
between  its  anterior  one-third  and  posterior  two-thirds.  It 
gives  lodgment  to  a  number  of  smooth  muscle  cells,  many 
of  which  are  arranged  longitudinally  to  the  long  axis  of  the 
eyeball.  In  front  of  the  ciliary  body,  the  choroid  terminates 
in  the  form  of  the  iris,  a  membranous  curtain  which  applies 
itself  very  closely  to  the  anterior  surface  of  the  lens,  so  that 
only  a  very  narrow  space  is  left  here  which  is  filled  with 
aqueous  humour. 

The  lens  of  the  human  eye  is  invested  by  a  capsule  which  in 
turn  is  connected  with  the  ciliary  body  by  means  of  ligament- 
ous  threads.  It  presents  a  biconvex  shape,  its  anterior  sur- 
face always  being  less  convex  than  its  posterior.  As  such  it 
possesses  the  power  of  gathering  the  rays  of  light  and  bring- 
ing them  to  a  sharp  point  of  intersection  behind  it.  The 
spot  in  which  these  rays  are  centralized,  is  called  the  focus. 
The  general  truth  here  alluded  to  is  familiar  to  nearly  every- 
body, because  if  a  biconvex  lens  is  held  at  a  certain  distance 


384  THE    SENSE-ORGANS 

from  a  dark  wall  and  a  candle  is  placed  in  front  of  it,  an 
image  will  be  formed  of  the  latter  upon  this  screen.  The 
image,  however,  is  upside  down,  because  the  rays  of  light  are 
inverted  by  the  lens. 

By  moving  the  candle  nearer  to  and  farther  away  from  the 
lens,  it  will  be  found  that  the  image  upon  the  wall  enlarges 
when  the  distance  is  decreased.  Furthermore,  it  will  be 
noted  that  a  perfectly  clear  image  is  obtained  only  when  the 
distances  between  the  candle,  lens,  and  screen  are  adjusted 
in  a  particular  way.  At  all  other  distances  the  image  be- 
comes blurred  for  the  reason  that  the  rays  of  light  are  not 
sharply  focalized  upon  the  wall.  It  is  a  well  known  fact  that 
far  and  near  objects  cannot  be  photographed  simultaneously. 
A  different  adjustment  of  the  lens  and  sensitive  plate  of  the 
camera  is  required  for  each.  One  of  two  methods  may  be 
followed  in  order  to  obtain  a  perfectly  clear  picture :  namely, 
(a)  the  lens  may  be  moved  forward  for  near  work  or  backward 
for  far  work,  and  (6)  the  plate  may  be  drawn  backward 
when  receiving  the  rays  of  a  near  object,  or  forward  when  a 
distant  object  is  to  be  photographed.  The  fact  that  the 
focal  distances  must  be  changed  in  this  way  everyone  can 
assure  himself  of  by  simply  placing  the  candle  at  different 
distances  from  the  screen  and  endeavoring  to  form  a  distinct 
image  of  it  by  changing  the  position  of  either  the  lens  or  the 
screen. 

The  Process  of  Accommodation. — There  is  still  another 
way  in  whi.;h  this  adjustment  could  be  effected,  namely,  to 
change  the  refractive  power  of  the  lens,  and  not  the  position 
of  the  lens  or  screen.  Clearly,  since  the  rays  of  light  emitted 
by  a  near  '  ^ject  must  be  more  quickly  converged  than  those 
coming  fro  L  far,  near  work  requires  a  lens  possessing  a  greater 
convexity  or  refractive  power.  Thus,  if  a  photographic 
camera  could  be  equipped  with  a  rotary  disc  carrying  a  num- 
ber of  biconvex  lenses  of  different  power  it  would  fulfill  its 
purpose  as  well  as  one  in  which  a  single  lens  is  moved  forward 
and  backward.  The  former  type  of  camera,  however,  is 
more  expensive  to  manufacture  and  less  convenient  to 
handle;  and  hence,  is  of  slight  value  commercially. 

Excepting  the  amphibia,  the  eyes  of  all  the  higher  animals 


THE    RAYS    OF    LIGHT    THROUGH    THE    EYE 


385 


contain  a  mechanism  by  means  of  which  their  focal  power 
may  be  altered  so  as  to  enable  them  to  form  an  image  of  near 
as  well  as  distant  objects  upon  their  retinae.  The  manner 
in  which  this  end  is  accomplished  differs  in  different  animals. 
Thus,  the  eyes  of  the  fish  are  set  for  near  objects,  and  distant 
objects  are  focalized  by  them  by  retracting  the  lens  by  means 
of  a  special  muscle  attached  to  its  peripheral  zone.  Certain 


FIG.  143. — Diagram  illustrating  the  process  of  accommodation  in  the 
human  eye.  C,  cornea;  L,  lens;  /,  iris;  CL,  ciliary  ligament;  CB,  ciliary 
body;  Ch,  choroid;  R,  retina;  S,  sclera.  On  near  vision  the  ciliary  muscle 
contracts,  drawing  the  region  B  nearer  to  region  A.  The  tension  upon  the 
ciliary  ligament  being  diminished  thereby,  the  lens  assumes  a  more 
spherical  shape,  chiefly  in  the  direction  of  the  cornea.  This  change  is 
indicated  in  red. 

molluscs  shorten  their  eyeballs  in  the  manner  of  a  folding 
camera,  so  that  the  retinae  are  actually  brought  nearer  the 
lens.  In  brief,  it  may  be  stated  that  all  the  physical  methods 
of  accommodation  outlined  above  find  representation  in  the 
animal  kingdom.  The  eyes  of  the  mammals  are  ordinarily 
set  for  far  objects.  Near  objects  are  accommodated  for  by 
them  by  rendering  the  lens  more  conyex,  thereby  increasing 
its  refractive  power.  This  method  of  accommodation  cor- 
responds in  a  way  to  that  physical  system  which  permits  us  to 
substitute  lenses  of  varying  convexity. 

25 


386  THE    SENSE-ORGANS 

The  manner  in  which  the  lens  of  the  mammalian  eye  is  ad- 
justed for  near  vision,  has  been  satisfactorily  explained  in 
accordance  with  the  "  theory  of  detention."  It  is  believed 
that  the  lens  is  ordinarily  held  under  a  certain  degree  of 
tension,  which  is  imposed  upon  it  by  the  ciliary  ligaments 
and  body.  At  this  time  it  is  relatively  flat  and  set  for  far 
objects.  On  near  vision  the  ciliary  muscles  contract,  thereby 
slightly  displacing  the  ciliary  body  in  a  forward  direction. 
This  releases  the  tension  upon  the  ciliary  ligaments  and  sub- 
stance of  the  lens,  and  allows  the  latter  to  assume  a  more 
convex  outline. 

The  Near  Point  of  the  Eye. — When  the  eyeball  possesses  a 
normal  shape,  and  its  different  parts  are  thoroughly  elastic, 
the  lens  is  always  able  to  intersect  the  rays  of  light  upon  the 
retina.  But  this  rule  holds  true  only  for  those  objects 
which  are  situated  between  the  horizon  and  the  near  point  of 
the  eye.  It  is  only  natural  to  suppose  that  if  an  object  is  held 
very  near  the  eye,  the  power  of  the  ciliary  mechanism  must 
finally  fail  to  impart  to  the  lens  a  convexity  sufficient  to 
converge  the  rays  upon  the  retina?.  In  the  normal  eye  at 
the  age  of  twenty  years,  this  point  lies  10  cm.  in  front  of  the 
cornea.  Consequently,  any  object  situated  inside  the  near 
point  cannot  be  accurately  focalized  and  yields,  therefore,  a 
blurred  image.  As  we  grow  older,  the  near  point  moves 
outward  until  at  about  forty  years  of  age  it  lies  22  cm.  from 
the  cornea.  General  infiltrations  and  senile  changes  are 
responsible  for  its  displacement.  * 


CHAPTER  XL 


THE  STIMULATION  OF  THE  RETINA  BY  THE  RAYS 
OF  LIGHT 


r 


The  Structure  of  the  Retina. — The  retina  covers  practically 
the  entire  concavity  of  the  poste- 
rior chamber  of  the  eyeball.  It 
terminates  directly  behind  the 
base  of  the  'ciliary  body,  forming 
here  a  line  of  demarcation  which 
is  termed  the  ora  serrata.  Under 
the  high  power  of  the  micro- 
scope it  presents  several  layers 
of  nervous  elements,  of  which 
the  rods  and  cones  are  the  most 
important.  These  cells  consist 
of  two  segments,  which  are  desig- 
nated respectively  as  their  outer 
and  inner  limbs.  The  former  lie 
deeply  embedded  in  the  pigment 
cells  of  the  choroid  coat,  while 
the  latter  are  directed  toward 
the  vitreous  humour.  Further- 
more, while  the  outer  limbs  of 
the  rods  are  cylindrical  in  shape 
and  exhibit  definite  cross-stria- 
tions,  those  of  the  cones  possess 
a  conical  outline  and  pointed 
extremity. 

The  rods  and  cones  are  sur- 
mounted by  several  additional 
strata  of  nervous  tissue  which 
are  designated  as  the  outer 
nuclear,  outer  molecular,  inner 
nuclear,  and  inner  molecular. 

387 


i. 


n. 


FIG.  144.— I,  a  rod;  II,  a 
cone  of  mammalian  retina;  h, 
external  limiting  membrane. 
((?.  Greeff.) 


388  THE    SENSE-ORGANS 

Each  layer  embraces  a  certain  number  of  ganglion  cells,  pos- 
sessing different  shapes  and  positions.  The  fibers  arising  from 
these  elements  pass  inward  and  eventually  form  a  delicate 
sheet  between  the  vitreous  humour  and  the  inner  molecular 
layer,  which  is  marked  off  against  the  former  by  a  peculiar 
network  of  nervous  connective  tissue,  the  internal  limiting 
membrane.  Naturally,  these  nerve  fibers  arise  in  all  parts 
of  the  retina  and  strive  toward  a  common  point  of  exit  from 
the  eyeball,  which  is  known  as  the  optic  papilla.  Centrally 
to  this  point  they  form  the  optic  nerve. 

This  perforation  in  the  capsule  of  the  eyeball  also  serves 
as  the  point  of  entrance  for  the  bloodvessels  and  lymphatics 
of  the  retina,  which  ramify  principally  between  its  internal 
limiting  membrane  and  inner  nuclear  layer.  It  will  be  seen, 
therefore,  that  the  nerve  fibers  are  placed  in  front  of  the 
cellular  components  of  the  retina  and  in  the  direct  path  of  the 
rays  of  light.  But  since  this  arrangement  does  not  interfere 
with  vision,  it  must  be  concluded  that  a  sufficient  number 
of  light  rays  reach  the  underlying  rods  and  cones  in  spite  of 
these  obstacles.  The  entrance  of  the  optic  nerve  or  optic 
papilla  contains  solely  fibers  and  no  rods  and  cones,  nor  other 
cellular  elements.  Consequently,  inasmuch  as  this  particu- 
lar area  cannot  be  activated  by  the  rays  of  light,  it  is 
appropriately  called  the  blind-spot. 

The  Function  of  the  Retina. — The  foregoing  discussion 
must  have  shown  that  the  retina  possesses  the  power  of 
converting  the  ethereal  vibrations  into  nerve  impulses  which 
finally  attain  representation  in  consciousness  through  the 
agency  of  the  corresponding  center  in  the  cerebrum.  Vision 
is  the  product  of  the  center  and  not  of  the  retina.  The  latter, 
together  with  its  adjunct  structures,  merely  serves  as  the 
mediator  between  the  center  and  the  outside  world.  The 
process  by  which  this  transfer  is  effected,  is  not  clearly 
understood,  although  it  is  generally  assumed  that  the  retina 
acts  in  the  manner  of  a  sensitive  plate.  This  theory  implies 
that  the  rays  of  light  give  rise  to  a  reduction  of  certain 
pigments  which,  however,  are  immediately  rebuilt.  Upon 
this  basis,  the  optic  impulses  would  be  the  results  of  chemical 
interactions.  This  view  finds  support  in  the  fact  that  the 


THE    STIMULATION    OF   THE    RETINA  389 

retina  contains  a  pigment,  called  visual  purple,  which  may  be 
extracted  and  dealt  with  in  the  manner  of  any  reducible 
substance.  It  is  true,  however,  that  certain  animals  are 
not  in  possession  of  this  type  of  pigment,  and  nevertheless 
present  a  perfectly  normal  acuity  of  vision.  This  fact,  in 
conjunction  with  others  to  be  described  later,  leads  us  to  infer 
that  the  rods  and  cones  themselves  are  sensitive  to  light,  and 
employ  the  visual  purple  merely  as  an  activating  substance 
to  intensify  the  rays  of  low  striking  force. 

The  Blind  Spot. — A  simple  experiment  which  may  be 
performed  to  prove  that  the  optic  papilla  is  insensitive  to  the 
rays  of  light,  is  the  following:  Close  your  left  eye  and  with 
your  right  eye  look  steadily  at  the  cross  of  Fig.  145,  held  at  a 
distance  of  about  20  cm.  from  its  cornea.  Now,  move  the 


FIG.  145. — Diagram  to  demonstrate  presence  of  blind  spot  in  the  visual 
field.  Fix  the  cross  with  the  right  eye;  bring  figure  closer  to  eye  until  the 
white  dot  appears.  (Helmholtz.) 

figure  towards  you  until  the  black  dot  disappears.  At  this 
distance  the  rays  emitted  by  the  dot  fall  upon  the  papilla  of 
this  eye  and  are  not  perceived.  Naturally,  the  moving  of  the 
figure  still  closer  to  the  cornea  will  cause  the  dot  to  reappear, 
because  the  rays  previously  focalized  upon  the  blind  spot, 
must  then  leave  this  area  and  again  strike  the  retina  proper. 
When  both  eyes  are  used  in  vision,  this  fact  that  the  optic 
papillae  are  insensitive  to  light,  cannot  give  rise  to  visual 
disturbances,  because  while  the  rays  emitted  by  an  object 
may  fall  upon  the  blind  spot  of. one  eye,  they  must  then 
strike  the  outlying  districts  of  the  opposite  retina.  The  latter 
sensation  overcomes  the  defect  in  the  opposite  visual  field. 
In  this  connection,  brief  reference  should  also  be  made  to  the 
fact  that  the  rays  entering  the  eyes  under  normal  conditions, 
are  not  focalized  upon  anatomically  related  areas.  But, 


390  THE    SENSE-ORGANS 

although  the  visual  elements  involved  occupy  different  posi- 
tions in  the  two  retinae,  the  sensations  mediated  by  them 
are  perfectly  harmonious  in  their  character. 

The  Yellow  Spot. — About  3.5  mm.  to  the  outside  of  the 
blind  spot  lies  the  yellow  spot  or  macula  lutea,  which  forms 
the  most  sensitive  area  of  the  retina.  Structurally,  this 
region  is  differentiated  from  the  remaining  portion  of  the 
retina  by  the  fact  that  it  contains  solely  a  large  number  of 
closely-set  cones.  It  is  also  noted  that  these  elements  are 
here  directly  exposed  to  the  entering  rays  of  light,  while  else- 
where they  are  covered  by  several  layers  of  cell-bodies  and 
fibers.  Furthermore,  inasmuch  as  the  visual  purple  is  an 
adjunct  of  the  rods,  it  is  found  in  all  parts  of  the  retina  but 
not  in  the  yellow  spot.  The  diameter  of  this  area  measures 
2.0  mm.,  whereas  that  of  the  blind  spot  measures  1.8  mm. 

Functionally,  the  macula  lutea  is  differentiated  from  the 
remaining  regions  of  the  retina  by  the  fact  that  it  is  much 
more  sensitive  to,  light  than  the  others.  Thus,  clear  vision 
invariably  requires  us  to  bring  the  object  in  a  direct  line  with 
the  center  of  the  macula,  designated  as  the  fovea  centralis. 
This  end  is  accomplished  by  the  contraction  of  the  external 
muscles  of  the  eyeball.  In  dim  light,  on  the  other  hand,  we 
always  endeavor  to  focalize  the  object  upon  the  outlying 
districts  of  the  retina,  activating  thereby  the  rods  and  visual 
purple.  Thus,  direct  vision  is  mediated  by  the  cones  and  is 
employed  in  high  intensities  of  light,  while  the  rods  are  the 
elements  of  indirect  vision  and  are  activated  with  the  aid  of 
the  retinal  pigment  by  rays  of  low  striking  force. 

It  is  a  matter  of  common  experience  that  visual  impressions 
are  obtained  not  only  during  the  periods  of  retinal  stimula- 
tion but  persist  for  some  time  thereafter.  Furthermore,  the 
duration  of  the  initial  excitation  is  often  surprisingly  brief, 

because  a  flash  of  lightning  lasting  only  sec.,  is 

-L,UUU,UUU 

sufficient  to  evoke  a  sensation.  The  successive  stimulations, 
however,  must  be  separated  from  one  another  by  appreciable 
intervals,  otherwise  a  fused  impression  will  be  obtained. 
Thus,  a  luminous  rod  if  turned  around  at  a  steadily  increas- 
ing speed,  finally  yields  a  continuous  circular  visual  concept. 


THE    STIMULATION    OF   THE    RETINA  391 

This  principle  is  employed  in  cinematography  to  reproduce 
the  movements  of  objects.  A  series  of  instantaneous  photo- 
graphs taken  of  an  object  in  motion,  are  projected  in  quick 
succession  upon  a  screen,  so  that  the  succeeding  one  always 
produces  its  impression  before  that  of  the  preceding  one  has 
died  out.  In  order  to  accomplish  this  fusion  not  less  than 
ten  photographs  must  be  consecutively  projected  in  the  time 
of  one  second. 

Optical  Defects  of  the  Eye. — The  purpose  of  the  normal  eye 
is  to  bring  rays  of  light  to  a  precise  intersecting  point  upon 
the  retina.  An  eye  that  is  able  to  accomplish  this  end,  is 
said  to  be  emmetropic.  Accordingly,  the  condition  of  em- 
metropia  signifies  normal  vision.  Contrariwise,  an  eye  which 
is  unable  to  focalize  the  rays  of  light  precisely  upon  the  retina, 
is  characterized  as  ametropic.  This  condition  of  abnormal 
refraction,  which  is  known  as  ametropia,  may  be  due  to  the 
following  causes: 

(a)  imperfect  curvature  of  the  cornea,  astigmatism, 

(b)  diminished  elasticity  of  the  lens,  presbyopia, 
(c) imperfect  shape  of  the  eyeballs: 

1.  Myopia,  the  eyeball  is  too  long, 

2.  Hypermetropia,  the  eyeball  is  too  short. 

Probably  the  most  prevalent  condition  is  presbyopia,  which 
is  due  to  a  loss  of  the  elasticity  of  the  lens.  Usually  about 
the  forty-fifth  year  of  our  life,  certain  changes  begin  to  make 
themselves  felt  which  are  characterized  by  general  infiltra- 
tions of  our  tissues,  rendering  them  less  pliable.  The  crystal- 
line lens  does  not  form  an  exception  to  this  rule,  and  hence, 
it  is  noted  that  it  becomes  increasingly  inflexible.  For  this 
reason,  it  can  no  longer  be  rendered  so  convex  as  formerly.  A 
flat  lens  is  employed  to  focalize  distant  objects.  Conse- 
quently, the  presbyopfc  eye  is  unable  to  form  a  perfectly 
clear  image  of  near  objects.  This  difficulty  is  remedied  by 
placing  a  biconvex  lens  in  front  of  the  eye  whenever  near 
vision  is  required. 

It  is  true  that  practically  every  eye  is  slightly  astigmatic, 
because  the  different  prismatic  elements  of  its  lens  are  not 
arranged  as  in  a  perfect  optical  system.  This  "normal" 


392  THE    SENSE-ORGANS 

degree  of  astigmatism  does  not  give  rise  to  actual  distur- 
bances in  vision.  The  usual  cause  of  the  development  of 
astigmatic  aberrations  is  an  unequal  curvature  of  the  cornea, 
causing  a  distortion  of  the  bundle  of  light  as  it  traverses  this 
membrane.  Obviously,  that  segment  of  the  cornea  which 
possesses  the  greatest  convexity,  must  bring  the  entering 
rays  of  light  more  quickly  to  a  focus  than  the  less  convex.  As 
a  rule,  these  differences  in  the  curvature  of  the  cornea,  are 
placed  at  right  angles  to  one  another,  and  affect  chiefly 
its  horizontal  and  vertical  meridians. 

The  conditions  of  near-sightedness  or  myopia  ^^far-sighted- 
ness or  hypermetropia,  are  due  to  a  faulty  shape  of  the  eyeball, 


FIG.  146. — Diagram  to  illustrate  the  refraction  in  a  myopic  eye.  L, 
luminous  point  focalized  in  L1  in  the  vitreous  humor.  A  concave  lens 
L  renders  these  rays  more  divergent  so  that  they  are  made  to  intersect 
upon  the  retina  in  ZA 

arising  in  consequence  of  inherited  abnormalities  and  ac- 
quired errors  in  reading.  Thus,  the  eyeball  may  lengthen 
in  the  course  of  time  until  the  focal  point  of  the  light  rays 
comes  to  lie  in  the  vitreous  humor.  Behind  this  point,  the 
rays  again  diverge  and  finally  strike  the  retina  far  apart. 
In  this  way,  a  dispersion  circle  of  light  is  formed  upon  this 
receptor  which  cannot  yield  a  perfectly  clear  image.  Fu~c- 
thermore,  this  image  cannot  be  rendered  more  precise  by 
extra  efforts  at  accommodation,  because  all  greater  degrees  of 
convexity  of  the  lens  must  force  the  focal  point  farther  for- 
ward into  the  vitreous  humor,  thereby  increasing  the  size 
of  the  circle  of  dispersion  upon  the  retina. 


THE    STIMULATION    OF    THE    RETINA 


393 


A  person  whose  eyeballs  are  too  long,  cannot  form  a  clear 
image  of  distant  objects,  although  he  is  well  able  to  focalize 
near  objects.  For  this  reason,  this  condition  is  known  as 
near-sightedness  or  myopia.  In  order  to  remedy  it,  the  focal 
point  must  be  moved  farther  backward  until  it  strikes  the 
retina.  This  end  is  accomplished  by  placing  a  biconcave 
lens  in  front  of  the  cornea  which  presents  the  eye  with  rays 
of  light  more  divergent  than  they  would  be  otherwise. 

The  condition  of  far-sightedness  or  hypermetropia  arises 
when  the  eyeball  is  shorter  than  normal,  so  that  the  focal 


FIG.  147. — Diagram  to  illustrate  the  refraction  in  a  hypermetropic  eye. 
L,  luminous  point  focalized  in  Z/1  "behind"  the  retina.  A  convex  lens 
C  renders  these  rays  more  convergent  so  they  are  made  to  intersect  upon 
the  retina  in  L2. 

point  falls  behind  the  retina.  Consequently,  the  rays  of 
light  must  strike  the  retina  while  still  far  apart,  and  cannot, 
therefore,  give  rise  to  a  precise  image.  A  person  whose 
eyeballs  are  too  short,  cannot  see  objects  near  him  very 
distinctly,  although  he  can  accommodate  for  them  in  a 
measure  by  rendering  the  lens  especially  convex  by  making 
extra  efforts  at  accommodation.  This  condition  may  be 
remedied  by  placing  a  biconvex  lens  in  front  of  the  cornea 
which  renders  the  entering  rays  of  light  more  convergent 
than  they  would  be  otherwise.  This  enables  the  eye  by  its 
own  efforts  to  bring  the  focal  point  farther  forward  until  it 
comes  to  lie  precisely  upon  the  retina. 


INDEX 


ABDOMINAL  organs,  blood-supply 

of,  179 

Abducens  nerve,  349 
Abduction  and  adduction,  75 
Absorption,  255,  260 

intestinal  lining  cells  as  factors 
in,  261 

of  water,  movement  in  conse- 
quence of,  48 
Acceleration  of  heart,  174 
Accessory  nerve,  350 
Accommodation,  process  of,  384 
Accommodation-reflex,  383 
Acetone  in  urine,  291 
Acids,  fatty,  270 

hydrochloric,  237 
Acinus,  220,  228 


Acromegaly,  301 
Act  of  swalU 


lowing,  233 
Action  of  cardiac  valves,  139 
Active  movement  by  absorption 

of  water,  48 
Activity,  vital,  262 
Addison's  disease,  299 
Adduction    and    abduction,     75 
Adenoid,  228 
Adipose  tissues,  271 
Adrenal  glands,  299 
function,  300 
secretion  of,  300 
Adrenalin,  295,  300 

effect  of,   on  muscle   contrac- 
tion, 85 
Adrenin,  300 

Adult  mammalian  heart,  133 
Afferent  neurones,  95,  306 
Age,  effect  of,  on  arterial  blood- 
pressure,  165 
Air,  complemental,  201 
constituents  of,  182 
content    of    lungs,     effect    of 
respiratory    movements    on, 
200 


Air,  residual,  201 

respired,  changes  in,  201 
stationary,  201 
supplemental,  201 
tidal,  201 

Air-cells  of  lung,  190 
Albumin  in  urine,  290 
Albuminoids,  270 
Albuminous  glands,  229 
Albuminuria,  290 
Alimentary  canal,  223 

length  of,  in  various  animals, 

223 

mucosa  of,  224 
muscular  coat  of,  224 
peritoneum  of,  224 
serous  layer  of,  224 
wall  of,  224 
glycosuria,  268 

Altitudes,  high,  respiratory  inter- 
change at,  206 
low,  respiratory  interchange  at, 

208 

Alveoli  of  lung,  190 
Ametropia,  391 
Ametropic  eye,  391 
Amino-acids,  154,  238,  268,  269, 

270 

Amoeboid  motion,  50 
Amphibian  heart,  130 
Ampulla  of  semicircular  canals. 

371 

Amylopsin,  247 
Anabolism,  35 
Anaemia,  119 
Animal  heat,  272 
Animals  and  plants,  relationship 

between,  30 
cold-blooded,  279 
segmental,  307 
spinal  reflex,  311 
warm-blooded,    279 
Animate  material,  37 


395 


396 


INDEX 


Anterior  segment  of  gray  matter. 

316 
Anti-peristaltic  motions  of  large 

intestine,  256 
Anvil-bone  of  ear,  364 
Aorta,  128 
Aortic  valve,  138 
Apex  beat,  146 
Aphasia,  339 
Apnoea,  204 

Aqueduct  of  Sylvius,  320 
Aqueous  humour,  379 

effect  of,  on  passage  of  light 

through  eye,  381 
Arachnoid,  312 
Area,  body-sense,  336 

motor,  of  cerebrum,  333 
Arterial  blood-pressure.     See 

Blood-pressure,  arterial. 
pulse,  168 
cause,  168 
character  of,  sphygmograph 

for  studying,  169 
dicrotic  wavelet  of,  170 
frequency  of,  169 
hard,  170 
large,  170 
rapid,  170 
slow,  170 
small,  170 
soft,  170 

Arteries,  walls  of,  155 
Arterioles,  128 
Artery,  hepatic,  249 

renal,  285 
Asphyxia,  205 
Assimilation,  35,  216 
Association  center,  322 

realms,  development  of,  308 
reflexes,  319 

system,  of  cerebrum,  328 
Associations,  308 
Astigmatism,  392 
Asynergia    from    lacerations    of 

cerebellum,  343 
Atmospheric  pressure,  182 
Auditory    canal,     external,     363  j 

nerve,  349 

Auriculo-temporal  nerve,  349 
Auscultation    method    of    deter-  i 

mining  blood-pressure,  164 
Autacoid  substances,  296 
Automatic  centers  of  medulla  ob- 
longata,  345 


Automatic  tissue,  58 
Autonomic  nervous  system,  304, 

350 
parasympathetic    division 

of,  351 
sympathetic    division    of, 

351 
Axones,  93 

BASAL  consumption,  276 

metabolism,  276 
Basedow's  disease,  298 
Basilar  membrane,  368 
Bends,  208 
Betz,  cells  of,  326 
Bile,  251 

digestive  function  of,  252 

duct,  common,  252 

storage  of,  252 
Bile-salts,  251 
Bilirubin,  251 
Biliverdin,  251 
Biology,  scope,  19 
Bismuth  x-ray  study  of  stomach, 

242 

Bleeder,  124 
Blindness,  half,  338 
Blind-spot,  388,  389 
Blood,  115 

characteristics,  115 

circulation  of,  127 

coagulation  of,   123.     See  also 
Coagulation  of  blood. 

color  of,  115,  119 

composition  of,  115 

corpuscles,  red,  117 
appearance,  117 
number  of,  118 
of    cold-blooded    animals. 

118 
white,  120 

amoeboid  qualities  of,  121 
size,  120 

distribution  of,  116 

flow  of,  157 
speed  of,  157 

function  of,  126 

hemoglobin  of,  119 

odor  of,  116 

platelets,  122 

specific  gravity  of,  116 

taste  of,  116 

temperature  of,  116 

total  quantity  of,  116 


INDEX 


397 


Blood-pressure,  arterial,  effect,  of 

age  on,  165 
of  eating  on,  167 
of    muscular   exercise    on, 

166 

of  posture  on,  166 
of  sleep  on,  167 
of  temperature  on,  167 
height  of,  165 
differences  in,  160 
methods  of  determining,  162 
by  auscultation,  164 
by  palpation,  164 
direct,  163 
indirect,  163 
related  phenomena,  160 
Blood-supply  of    abdominal    or- 
gans ,179 

Blood-vessels,  caliber  of,  regula- 
tion of,  175 
constriction  of,  176 
dilatation  of,  176 
nervous  control  of,  171 
structure  of, 

Blushing,  phenomenon  of,  177 
Body,  metabolic  requirements  of, 

272 

energy  requirements  of,  276 
temperature,  278 

regulation  of,  279 
Body-fluids,     characteristics     of, 

107 

Body-sense  area,  336 
Bolus,  226 

Bomb-calorimeter,  272 
Bones  as  aid  to  muscular  power, 

72 

ear,  364 

Bowman,  capsule  of,  287 
Brain,  307,  308,  320 
complex,  323 
general    arrangement    of, 

320 

simple,  321 
weight  of,  323 
Breath,  shortness  of,  213 
Breathing  exercises,  214 

necessity  of,  184 
Breathlessness,  212 
Bronchi,  190 
Bronchioles,  190 
Brunner,  glands  of,  246 
Bulb.     See     Medulla     oblongata. 
Bundle  of  His,  144 


CAISSON  disease,  208 
Calcarine  fissure,  328 
Caliber  of  blood-vessels,  regula- 
tion of,  175 
Calloso-marginal,    of    cerebrum, 

328 

Caloric     value     of     foods,     276 
Calorie,  274 

large,  275 

small,  275 
Calorimeter,  273 
Calorimetry,  272 
Canal,  alimentary,  223 

central,  of  ear,  368 

vertebral,  312 

Canals,  semicircular,  368,  370 
Capillaries,  129 

walls  of,  155 
Capsule,  285 

of  Bowman,  287 
Carbohydrate-fat,  267 
Carbohydrates,  27 

life  history  of,  265 

metabolism,  impaired,  267 
Cardia,  235 
Cardiac  center,  172,  345 

cycle,  136 

dyspnoea,  214 

impulse,  146 

muscle  tissue,  64 

nerves,  172 

sphincter  of  stomach,  236 

valves,  action  of,  139 
Cardio-acceleration  of  heart,    172 
Cardie-inhibition    of   heart,    172 
Cartilage,  cricoid,  190 
Casein,  238 
Caseinogen,  238 
Catabolism,  35 
Cavity  of  internal  ear,  365 

of  middle  ear,  364 

otolithic,  371 
Cell,  24 

carbohydrates  of,  27 

chemical  composition,  27 

chief,    of   gastric   glands,    239 

cytoplasm  of,  25,  27 

demilune,  229 

fats  of,  27 

hair,  of  ear,  369 

function  of,  370 

hepatic,  249 

inorganic  material  of,  27 

lipoids  of,  27 


398 


INDEX 


Cell,  metabolism  of,  35 

movement  of,  28 

by  changes  in  turgor,  48 

muscle,  smooth,  57 

of  Betz,  326 

of  Purkinje,  341 

olfactory,  359 

organic  material  of,  27 

oxidation  in  ,34 

parietal,  of  gastric  glands,  239 

proteins  of,  27 

salts  of,  27 

shape  of,  28 

size  of,  28 

water  of,  27 

Cell-body  of  nervous  tissue,  93 
Celom,  108 

Center,    automatic    of    medulla 
oblongata,  345 

cardiac,  345 

for  hearing,  338 

for  sight,  337 

for  smell,  338,  347 

for  speech,  339 

for  taste,  338 

for  writing,  339 

respiratory,  210,  347 

salivary,  230 

vasomotor,  347 
Central  canal  of  ear,  368 
Cerebellum,  309,  320,  341 

asynergia  from  lacerations  of, 
343 

function  of,  342 

location,  341 

molecular  layer  of,  341 

nuclear  layer  of,  341 

peduncles  of,  342 

size,  341 

structure  of,  341 

vermis  of,  341 

weight,  341 

Cerebral  cortex,  regions  of,  327 
Cerebro-spinal  fluid,  321 
Cerebrum,  309,  320,  322,  327 

gray   matter   of,    arrangement 
and  structure  of,  324 

hemispheres  of,  327 

lobes  of,  327 

localization  of  function  in,  333 

motor  area  of.  333 

removal  of,  328 

white  matter  of,  arrangement 
and  structure  of,  324 


Cerumen,  294 
Chalones,  296 
Cheeks,  226 
Chemical  energy,  42 

stimuli,  44 
Chemicals,   effect  of,   on  muscle 

contraction,  85 
Chemistry  of  contracting  muscle, 

87 

of  muscle,  86,  87 
of  rigor  mortis,  90 
Chilliness,  282 
Chlorosis,  119 
Cholesterin,  251 
Chorda  tympani,  230 
Chordae  tendine®,  140 
Choroid,  378 
plexus,  321 
Chyle,  114 
Chyme,  233 
Cilia,  51,  52 
Ciliary  body,  379 

motion,  51 
Circulating  fat,  271 
Circulation,  basic  principles  of, 

127 

of  blood  and  lymph,  107 
Circulatory    and   respiratory 
mechanisms,      co-ordination 
•  between,  212 
system,  151 

coronary  circuit  of,  154 
greater  circuit  of,  151 
lesser  circuit  of,  151 
normal,  128 
parts  of,  129 

pulmonary  circuit  of,  151 
systemic  circuit  of,  151 
Circumductioiyand  rotation,   75 
Classification  of  stimuli,  42 
Clotting  of  blood,  123.     See  also 

Coagulation  of  blood. 
Coagulation  of  blood,  123 
extravascular,  123 
intravascular,  123 
time  of,  123,  124 
Coagulation-time,  124 
Coagulum,  124 
Cochlea  of  ear,  368 
Cold,   effect  of,   on   muscle  con- 
traction, 85 

Cold-blooded  animals,  279 
Collapse  of  lung,  193 
Column se  carneae,  140 


INDEX 


399 


Commissural     system     of     cere- 
brum, 328 

Common  bile  duct,  245 
Complemental  air,  201 
Complex  brain,  323 

heart,  133 

Concha  of  external  ear,  363 
Conduction,  spinal  cord  as  organ 

of,  314 
Conductivity  of    living    matter, 

38 

Cones   and   rods   of   retina,    387 
Congestion,  180 
Conservation     of     matter     and 

energy,  law  of,  31 
Constriction  of  blood-vessels,  176 
Consumption,  basal,  276 
Contractility  of  living  matter,  39 
Contracting    muscle,     chemistry 

of,  87 

Contraction  and  expansion,  pro- 
toplasmic, movement  by,  50 

height  of,  factors  modifying,  83 

muscular,  analysis  of,  76 
chemistry  of,  87 

of   muscle,    67,    76.     See    also 
Muscle  contraction. 

summation  of  muscle,  80 

tetanic,  81 

wave  of  muscle,  71 
Convolutions  of  brain,  323 
Cords,  vocal,  189 
Cornea,  379 

effect  of,  on  passage  of  light 

through  eye,  381 
Coronary    circuit    of    circulatory 
system,  154 

sinus,  155 

Corpus  callosum,  327 
Corpuscles,  115 

Malpighian,  288 

of  Grandry,  355 

of  Herbst,  355 

of  Vater-Paccini,  355 

red,  117.     See  also  Blood  cor- 
puscles, red. 

tactile,  354 

white,     120.     See    also    Blood 

corpuscles,  white. 

Corpuscular  theory  of  light,  375 
Corti,    organ    of,    365,    368,    369 

activation  of,  370 
Costal   type   of   respiration,    197 
Coughing,  189 


Cranial  nerves,  347 

course  and  function  of,  347 
Cretinism,    297 
Cretins,    298 
Cricoid  cartilage,  190 
Crista  acustica,  371 
Crus  cerebri,  328 
Crystalline  lens,  383 
'Curd,  238 
Cutaneous  sensations,  352 

general  considerations,  352 
Cycle,  cardiac, 

respiratory,  186 
Cystic  duct,  252 

DECEREBELLATE  animals,  loss  of 
muscular  co-ordination  in,  343 

Decussation,  pyramidal,  335 

Defecation,  257 

Deficiency  disease,  241 

Deflation  of  lung,  196 

Deglutition,  233 

Demilune  cell,  229 

Dendrites,  93 

Dermis,  292 

Dextrose,  265 

Diabetes  mellitus,  221,  246,  268, 
301 

Dialysis,  262,  264 

Diaphragm,  192,  195 

Diaphragmatic   type  of  respira- 
tion, 197 

Diastole,  136 

Dicrotic  wavelet  of  arterial  pulse, 
170 

Diffusion,  182,  262 
pressure,  182 

Digestion,  intestinal,  245 
purpose  of,  218 
salivary,  223 

Dilatation  of  blood-vessels,  176 

Direct  vision,  390 

Dissimilation,  35,  216 

Diver's  palsy,  208 

Drugs,  effect  of,  on  muscle  con- 
traction, 85 

Drum,  ear,  363 

Duct,    bile,    common,    245,    252 
cystic,  252 
hepatic,  252 
of  Ravinus,  228 
of  Wirsung,  245,  252 
Stenson's,  228 
Wharton's,  228 


400 


INDEX 


Dudgeon  sphygmograph,  169 
Dura  mater,  312 
Dwarfism,  301 
Dynamic  sense,  345,  374 
Dyspnoea,  204 
cardiac,  214 

EAR,  anvil-bone  of,  364 

bones,  364 

cochlea  of,  368 

drum,  363 

function  of,  364 
oscillation  of,  365,  366 

external,  362 
concha  of,  363 
pinna  of,  363 

hair  cells  of,  369,  370 

hammer-bone  of,  364 

internal,  367 
cavity  of,  365 

middle,  364 
cavity  of,  364 

stirrup-bone  of,  364 
Eating,     effect    of,    on    arterial 

blood-pressure,  167 
Effector,  100 

Efferent  neurones,  95,  306 
Elasticity  of  muscle,  70 
Electrical    energy    liberated    by 
beating  heart,  148 

stimuli,  44 

Electrocardiograph,  150 
Elementary  heart,  129 

lung,  181 

Emission  theory  of  light,  375 
Emmetropia,  391 
Emmetropic  eye,  391 
Emotional  glycosuria,  300 
Emulsification  of  fat,  247 
Endocardium,  144 
Endocrine  organs,  296 

classification  of,  295 
Endolymph,  368 
Energy,  chemical,  42 

in  food,  275 

manifestations  of,  42 

production    of,    in    muscle,    90 

requirements,  of  body,  276 

vibratory,  42 
Enterokinase,  246, 
Enzymes,  222 

function  of,  222 

of  pancreatic  juice,  246 
Epicardium,  145 


Epidermis,  292 
Epiglottis,  189 
Epilepsy,  336 

Epithelium,  sensory  of  eye,  376 
Equilibration,  344,  345 
Equilibrium,   organ  of,  in  lower 
animals,  370 

sense  of,  350,  362,  368 
Erepsin,  254 
Ergograph,  Mosso's,  76 
Erythroblasts,  118 
Erythrocytes,  115 
Escape  from  inhibition,  174 
Esophagus,  188 
Eupncea,  204 
Eustachian  tube,  365 
Evacuation  of  stomach  contents, 

243 

time  of,  243 
Excised  heart,  171 
Excitation  of  muscle,  67 
Excretion,  284 

channels  of,  284 
|   Excretory  organ,  skin  as,  292 
j   Exercises,  breathing,  214 
|  Expansion  and  contraction,  pro- 
toplasmic, movement  by,  50 

of  lungs,  193 
Expiration,  186 
Extension  and  flexion,  75 
External     auditory     canal,     363 

ear,  362 

muscles  of  eyeball,  379 

respiration,  183 
Exteroceptors,  354 
Eye,  ametropic,  391 

course  of  rays  of  light  through, 
381 

emmetropic,  391 

hypermetropic,  393 

myopic,  392 

near  point  of,  386 

optical  defects  of,  391 

pupil  of,  382 

sensory  epithelium  of,  376 

structure  of,  377 
Eyeballs,  377 

external  muscles  of,  379 

wall  of,  378 
Eye-spots,  376 

FACIAL  nerve,  349 
Far-sightedness,  392,  393 
Fat,  27 


INDEX 


401 


Fat,  circulating,  271 

emulsification  of,  247 

life  history  of,  270 

stored,  271 

tissue,  271 

Fatigue,    effect    of,    on    muscle 
contraction,  82 

of  muscle,  86 

substances,  88 
Fatty  acids,  270 
Faucial  tonsils,  227 
Feces,  formation,  259 
Fenestra  ovalis,  365 

rotunda,  365 
Ferment,  221 
Fermentation,  221 
Fever,  282 

breaking-point  of,  283 
Fibers  of  Purkinje,  144 
Fibrin,  124 
Fibrinogen,  124 
Filum  terminate,  313 
Fissure,  calcarine,  328 

great  longitudinal  of  cerebrum, 
327 

of  Rolando,  327 

of  Sylvius,  327 
Flagella,  51 

Flexion  and  extension,  75 
Fluid  pleural,  192 
Focus,  383 
Food,  36,  216,  218 

caloric  value  of,  276 

changes  in,  effected  in  mouth, 
225 

energy  in,  275 

maceration  of,  227 

progress  of,  through  intestines, 

255,  258 

Food-stuffs,  36,  218 
Foramina,  intervertebral,  313 
Forced  breathing,  198 
Forebrain,  320 
Formation  of  lymph,  109 
Fovea  centralis,  390 
Fundus  of  stomach,  235 

GALL-BLADDER,  253 

Ganglia,  98 

Ganglion,  Gasserian,  348 

semilunar,  348 
Gases,    respiratory,    interchange 

of,  203 
Gasserian  ganglion,  348 

26 


Gastric  digestion,  233 
glands,  238 

chief  cells  of,  239 
parietal  cells  of,  239 
juice,  237 

active  agents  of,  237 
secretion  of,  240 
lipase,  238 
Gastrin,  241 
Giant  growth,  301 
Gills,  185 

Glands,  adrenal,  299 
albuminous,  229 
gastric,  238.     See  also  Gastric 

glands. 

Meibomian,  378 
mucous,  229 

secretory  product,  227 
of  Brunner,  246 
of  Lieberkiihn,  254 
of  mouth,  227 
parathyroid,  296 
parotid,  228 
pituitary,  301 
salivary,  228     See  also  Salivary 

glands.  • 
sebaceous,  293 
sublingual,  228 
submaxillary,  228 
sweat,  292,  293 
thymus,  302 
thyroid,  296 
Glomerulus,  288 
Glossopharyngeal  nerve,  350 
Glycerin,  270 
Glycogen,  154,  251,  266 
Glycosuria,  290 
alimentary,  268 
emotional,  300 
physiological,  268 
Goiter,  298 
Goose-flesh,  282 
Grandry,  corpuscles  of,  355 
Grave's  disease,  298 
Gray  matter,  cerebral,  arrange- 
ment   and    structure    of, 

324 

of  spinal  cord,  314 
Great     longitudinal     fissure     of 

cerebrum,  327 
omentum,  225 

Greater  circuit  of  circulatory  sys- 
tem, 151 
Growth,  movement  by,  49 


402 


INDEX 


HAIR  cells  of  ear,  369 

function  of,  370 
Half-blindness,  338 
Hammer-bone  of  ear,  364 
Hearing,  center  for,  338 

sense  of,  362 
Heart,  acceleration  of,  174 

beat,  regulation  of,  171 

beating,  electrical  energy  liber- 
ated by,  148 

cardie-acceleration  of,  172 

cardio-inhibition  of,  172 

complex,  133 

elementary,  129 

excised,  171 

human  position,  150 
size,  150 
weight,  150 

hypertrophy  of,  148 

inhibition  of,  173 

muscles  of,  64 
tissue  of,  58 

nerves  of,  172 

nervous  control  of,  171 

of    mammals,  136 

segments  of,  manner  of  excita- 
tion of,  143 

sounds  of,  145 

valves    of,    arrangement,    136 

valvular  lesions  of,  146 

vasomotor  reactions  in,  179 
Heat,  animal,  272 

dissipation,  regulation  of, 

280 

Heat-rigor  of  muscle,  85 
Heliotropism,  376 
Hemianopia,  338 
Hemiplegia,  335 
Hemoglobin,  119 
Hemophilia,  124 
Henle,  loop  of,  288 
Hepatic  artery,  249 

cells,  249 

duct,  252 

vein,  250 

Herbst,  corpuscles  of,  355 
High  altitudes,  respiratory  inter- 
change at,  206 
Hilum  of  kidney,  285 
Hindbrain,  309,  320 
His's  bundle,  144 
Hormones,  296 
Humour,  aqueous,  379 

vitreous,  379 


Hutchinson's    spirometer,    Win- 

trich's  modification  of,  200 
Hydrocephalus,  332 
Hydrochloric  acid,  237 

action  of,  237 
Hypermetropia,  392,  393 
Hypertonic  solution,  264 
Hypertrophy  of  heart,  148 
Hypoglossal  nerve,  350 

ICTERUS,  250 
Impulse,  cardiac,  146 

spreading  of,  317 
Inanimate  material,  37 
Incomplete  tetanus,  81 
Incus,  364 

Indican  in  urine,  290 
Indirect  vision,  390 
Inferior  dental  nerve,  349 
Infantilism,  297 
Inferior  maxillary  branch  of  trig- 

eminus  nerve,  349 
Infundibula,  190 
Inhibition,  escape  from,  174 

of  heart,  173 
Innervation    of   salivary   glands, 

229 

sympathetic,  230 
Insensible  perspiration,  293 
Inspiration,  186 
Intensity  of  stimulation,  45 
Interauricular  node,  144 
Intercostal  muscles,  action  of,  in 

respiration,  198 
Internal  ear,  367 
cavity  of,  365 

limiting   membrane   of  retina, 
388 

respiration,  184 

secretions,  295 

organs  of,  295 
Interoceptors,  354 

general,  354 

special,  354 

Intervertebral  foramina,  313 
Intestinal  digestion,  245 

juice,  254 

lining  cells  as  factors  in  absorp- 
tion, 261 

Intestine,   large,    movements   of, 
256 

small,  movements  of,  255 
Intestines,      progress      of      food 

through,  255,  258 


INDEX 


403 


Intralobular  vein,  250 
Invertase,  254 

Involuntary  muscle  tissue,  58 
Iris,  379 

effect  of,  on  passage  of  light 
through  eye,  381 

functions,  382 

Irritability  of  living  matter,   38 
Island  of  Reil,  328 
Islands  of  Langerhans,  246 
Isotonic  solution,  264 

JAUNDICE,  254 
Juice,  gastric,  237 
pancreatic,  220.     See  also 
Pancreatic  juice. 

KIDNEYS,  284 

capsule,  285 

cortex  of,  285 

medulla  of,  285 

pelvis  of,  285 
Kymograph,  77 

LACTEALS,  113,  114 

Langerhans,  islands  of,  246 

Larynx,  188 

Lateral  discs,  64 

Law   of   conservation   of  matter 

and  energy,  31 
Lens,  379 

crystalline,  383 
Leukocytes,  115 

protective  power  of,  122 
Lieberkiihn,  glands  of,  254 
Life,  general  conditions  of,  37 

phenomena,  30 
Light,  cause  of,  375 

corpuscular  theory,  375 

emission  theory,  375 

rays  of,  course  of,  through  eye, 

381 
stimulation  of  retina  by,  387 

sources  of,  375 

stimulation  of,  nature  of,  375 

undulatory  theory,  375 
Light-reflex,  383 
Lingual  nerve,  349 
Lipase,  gastric,  238 
Lipoids,  27 
Liver,  248,  300 

blood  flow  in,  154 

blood-supply  of,  248 

functions  of,  250 


Living  matter,  17 

conductivity  of,  38 
contractility  of,  39 
irritability  of,  38 
metabolism  of,  39 
peculiarities  of,  38 
reproduction  of,  39 
Lobe,  220 

of  lung,  191 
Lobes,  olfactory,  322 

optic,  323 
Lobule,  220 

of  lung,  191 

Localization  of  function  in  cere- 
brum, 333 

Locomotion,  movements  em- 
ployed in,  types,  74 
Locomotor  ataxia,  344 
Loop  of  Henle,  288 
Low  altitudes,  respiratory  inter- 
change at,  208 
Lungs,  190 

air  content  of,  effect  of  respira- 
tory movements  on,  200 
air-cells  or  alveoli  of,  190 
collapse  of,  193 
deflation  of,  196 
elementary,  181 
expansion  of,  193 
mammalian,  190 
simple,  development  of,  183 
total  capacity,  201 
vital  capacity  of,  201 
Luxus  consumption,  272 
Lymph,  107 

circulation  of,  107 
flow  of,  114 

differences   in    pressure    in, 

114 

factors    controlling,  114 
lymphatic  valves  in,  114 
lymph-hearts  in,  114 
muscular      movements     in, 

114 

suction-action  in,  114 
formation  of,  109 
function  of,  126 

Lymphatic     channels,     distribu- 
tion of,  110 
valves,  114 

Lymph-corpuscles,  111 
Lymph-hearts,  114 
Lymph-nodes,  111 
Lymphocytes,  111 


404 


INDEX 


MACERATION  of  food,  227 
Macula  lutea,  390 
Malleus,  364 

Malpighian  corpuscle,  288 
Maltase,  254 
Maltose,  231,  265 
Mammalian  lung,  190 
Mammals,  heart  of,  136 

respiratory  mechanism  of,  188 
Manifestations     of     energy,     42 
Mandible,  226 
Manometer,  162 
Mastication,  226 

muscles  of,  226 

seat  of,  226 
Maximal  stimuli,  43 
Meatus,  urinary,  291 
Mechanical  stimuli,  43 
Medulla  oblongata,  323,  341,  345 
as  organ  of  conduction,  345 
automatic  centers  of,  345 
reflex  and  automatic  action 

of,  345 

Medullary  sheath  of  nerve,  97 
Meibomian  glands,  '378 
Membrane  of  Reissner,  369 

tympanic,  363 
Mesentery,  225 
Metabolic  requirements  of  body, 

272 
Metabolism,  35,  217,  265 

basal,  276 

carbohydrate,  impaired,  267 

of  living  matter,  39 
Metazoa,  55 
Micturition,  291 
Middle  ear,  364 

cavity  of,  364 
Minimal  stimuli,  44 
Mitral  valve,  138 
Moderator  band,  141 
Mosso's  ergograph,  76 
Molecular  layer  of  cerebellum,  341 
Motion,  46.     See  also  Movement. 
Motor    area    of    cerebrum,    333 

neurones,  95,  306 
Motor -plate,  100 
Mountain  sickness,  207 
Mouth,  changes  in  food  effected 
in,  225 

glands  of,  227 
Movement,  46 

active,  by  absorption  of  water, 
48 


Movement,  amoeboid,  50 
by  changes  in  cell  turgor,  48 

in  specific  gravity,  49 
by  growth,  49 
by    protoplasmic     contraction 

and  expansion,  50 
by  secretion,  49 
ciliary,  51 
effective  stroke,  53 
employed  in  locomotion,  types 

of,  74 

muscular,  55 
of  large  intestine,  256 
of  small  intestines,  255 
of  stomach,  242 
passive,  47 

respiratory,  regulation  of,  211 
types,  47 
Mucin,  227,  251 
action  of,  252 
Mucous  glands,  229 
of  mouth,  227 
secretory  product,  227 
Muscle,  46 
body,  61 
cells,  smooth,  57 
chemistry  of,  86,  87 
contracting,    chemistry   of,   $7 
contraction  of,  67, 
analysis  of,  76 
effect  of  cold  on,  85 

of  drugs  and  chemicals  on, 

85 
of  duration  of  stimulus  on, 

84 

of  fatigue  on,  82 
of  increasing  weights  on, 

84 

of  muscle  substance  on,  84 
of  strength  of  stimulus  on, 

83 

of   warmth    on,    84 
of  veratrin  on,  85 
height  of,  factors  modifying, 

83 

summation,  80 
tetanic,  81 

effect  of  fatigue  on,  86 
excitation  of,  67 
direct  activation  of,  69,  78 
fascia,  61 
fatigue,  82,  86 
general  behavior,  59 
heat-rigor,  85 


INDEX 


405 


Muscle,    indirect   activation   of, 

69,  78 
point  of  attachment,  61 

of  insertion,  61 
production  of  energy  in,  90 
rigor  caloris  of,  85 
simple  twitch,  76 
striated,  57 

principal  parts,  61 
substance   effect   of,    on    con- 
traction, 84 
tensor  tympani,  364 
tissue,  carctiac,  64 

elasticity  of,  70 

involuntary,  58 

of  heart,  58 

skeletal,  57 

smooth,  59 

striated,  60 

structure,  59 

visceral,  58 

voluntary,  57 
tonus,  69 

Muscle-sense,  origin  of,  345 
Muscles,  external,  of  eyeball,  379 
intercostal,   action  of,  in  res- 
piration, 198 
of  mastication,  226 
papillary,  140 
Muscular    contraction,    analysis 

of,  76 
co-ordination,  loss  of,  in  decer- 

ebellate  animals,  343 
exercise,  effect  of,   on  arterial 

blood-pressure,  166 
movements,  55 
power,  bones  as  aid  to,  72     • 
Myocardium,  145 
Myoids,  56 
Myopia,  392,  393 
Myxedema,  297 

NASAL  cavity,  359 
Near-point  of  eye,  386 
Near-sightedness,  392,  393 
Necessity  of  breathing,  184 
Nerve,  46,  306 

abducens,  349 

accessory,  350 

auditory,  349 

auriculo-temporal,  349 

effector,  100 

end-organ  of,  69 

facial,  349 


Nerve  ganglia,  98 

glossopharyngeal,  350 
hypoglossal,  350 
impulse  and  reflex  action,  93 
inferior  dental,  349 
lingual,  349 
neurilemma,  97 
oculomotor,  347 
of  sight,  347 
olfactory,  347 
optic,  347,  388 

physiological  properties  of,  101 
pneumogastric,  350 
receptor,  100 
trigeminal,  348 
trochlear,  348 
vagus,  350 
Nerves,  306 
cranial,  347 

course  and  function  of,  347 
distal  and  central  end-organs 

of,  97 

formation  of,  97 
medullary  sheath,  97 
splanchnic,  179 
vasomotor,  175 
Nervous    control    of    heart    and 

blood-vessels,  171 
regulation  of  respiration,  210 
system,  304 

autonomic,  304,  350 
centers  of,  306 
classification,  305 
functional  development,  304 
general  arrangement,  304 

function  of,  305 
parasympathetic,  305 
simple,  306 
sympathetic,  305 
tissue,  structure  of,  93 
Neurilemma  of  nerve,  97 
Neurone,  afferent,  95,  306 
efferent,  95,  306 
motor,  95,  306 
sensory,  95,  306 
Nissl's  granules,  93 
Node,  interauricular,  144 
Normal  breathing,  198 
Nuclear  layer  of  cerebellum,  341 
Nutrition,  216,  217 
Nuclei,  306 
Nucleolus,  26 
Nucleus,  26,  98 
Nutrition,  216,  217 


406 


INDEX 


OCULOMOTOR  nerve,  347 
Olfactory  cells,  359 

lobes,  322 

nerve,  347 

Omentum,  great,  225 
Opaque  bodies,  375 
Ophthalmic  branch  of  trigeminus 

nerve,  348 
Optic  lobes,  323 

nerve,  347,  388 

papilla,  388 

thalami,  328 

Optical  defects  of  eye,  391 
Optimum  stimuli,  44 
Ora  serrata,  387 

Organ    of    Corti,  365,    368,    369 
activation  of,  370 

of     equilibrium,     in     lower 

animals,  370 
Organism,  23 
Osmometer,    263 
Osmosis,  262,  263 
Osmotic  pressure,  263 
Ossicles,  364 
Otolithic  cavity,  371 
Otolyths,  371 
Oxidation,  33 

in  cells,  34 
Oxy-hemoglobin,  119 


PACCHIONIAN  bodies,  321 
Pace-maker  of  heart,  144 
Pain  sense,  357 
Palate,  hard,  226 

soft,  226 

Paling,  phenomenon  of,  177 
Palpation  method  of  determining 

blood-pressure,  164 
Palsy,  diver's,  208 
Pancreas,  245,  300 

secretion  of,  220,  245 
Pancreatic  juice,  220 

action  of,  246 

character  of,  246 

enzymes  of,  246 

secretion  of,  247 
Panniculus  adiposus,  271,  292 
Papilla,  optic,  388 
Papillary  muscles,  140 
Parasympathetic  division  of 

autonomic  system,  351 
Parathyroid  gland,  296 
Paresis,  general,  331 


Parietal  cells  of  gastric  glands 
239 

pleura,  192 

Parieto-occipital  groove,  327 
Parotid  gland,  228 
Passive  movement,  47 
Patellar  reflex,  318 
Peduncles  of  cerebellum,  342 
Pendular     movement    of    small 

intestine,  255 
Pepsin,  222,  238 

action  of,  238 
Pepsinogen,  222 
Peptones,  238 
Perception  reflexes,  319 
Pericardial  sac,  145 
Pericardium,  145 
Perilymph,  367 
Perimysium,  60 
Peristalsis,  regular,  256 
Peristaltic  waves,  60,  224 

movement   of  small  intestine, 

255 
Perspiration,  insensible,  293 

sensible,  293 
Phagocytosis,  121 
Pharyngeal  tonsil,  228 
Pharynx,  188,  226 
Phenomenon  of  stimulation,  40 
Phosphenes,  377 
Photic  stimuli,  44 
Phototaxis,  376 
Phrenology,  333 
Physiological  glycosuria,  268 

properties  of  nerve,  101 
Physiology,  definition,  17 

scope,  17 
Pia  mater,  312 
Pillars  of  fauces,  226 
Pinna  of  external  ear,  363 
Pituitary  gland,  301 
secretion  of,  301 
Plants  and  animals,  relationship 

between,  30 
Platelets,  blood,  122 
Pleura,  192 

parietal,  192 

visceral,  192 
Pleural  fluid,  192 
Pneumogastric  nerve,  350 
Point,  near,  of  eye,  386 
Portal  organs,  blood-supply    of, 
153 

vein,  153,  249 


INDEX 


407 


Posterior  horn   of  gray   matter, 

316 
Posture,    effect    of,    on    arterial 

blood-pressure,  166 
Presbyopia,  391 
Pressure,  atmospheric,  182 

diffusion,  182 

osmotic,  263 
Projection  system,  311 

of  cerebrum,  328 
Proprioceptors,  354 
Proteins,  27 

life  history  of,  268 
Protoplasm,  20 

origin     of     evolution     of,     21 
Protoplasmic     contraction     and 

expansion,    movement  by,    50 
Ptyalin,  226 

action  of,  231 
Pulmonary  semilunar  valves,  139 

veins,  132 
Pulse,  175 

arterial,  168.     See  also  Arterial 
pulse. 

venous,  170 
Pulse-pressure,  164 
Purkinje,  cells  of,  341 

fibers,  144 
Pupil  of  eye,  382 
Purple,  visual,  389 
Pyloric  sphincter  of  stomach,  236 

vestibule,     sphincter    of,     236 
Pylorus,  235 

Pyramidal  decussation,  335 
Pyramids,  288 

RADIATION,  281 

Ravinus,  duct  of,  228 

Rays    of    light,    stimulation    of 

retina  by,  387 
through  eye,  course  of,  381 
Receptors,  100 

of  sensations,  classification  of, 

354 

somatic,  354 
visceral,  354 
Reflection  of  light,  375 
Reflex,  accommodation,  383 
action,  103 

and  nerve  impulse,  93 
examples  of,  318 
spinal  cord  as  organ  of,  316 
circuit,  307 
light,  383 


Reflex,  patellar,  318 

time,  317 
Reflexes,  association,  319 

perception,  319 

spreading  of,  317 
Refraction  of  light,  375 
Regio  plfactoria,  360 

respiratoria,  359 
Regular  peristalsis,  256 
Regurgitation,  147 
Reil,  island  of,  328 
Reissner,  membrane  of,  369 
Renal  artery,  285 

vein,  285 
Rennet,  238 
Rennin,  238 

action  of,  238 
Reproduction    of  living  matter, 

39 

Reptile  heart,  130 
Residual  air,  201 
Respiration,  181 

action    of   intercostal   muscles 
in,  198 

calorimeter,  274 

chemistry  of,  200 

costal  type  of,  197 

diaphragmatic    type    of,     197 

external,  183 

forced,  198 

internal,  184 

mechanics  of,  188 

nervous     regulation     of,     210 

normal,  198 

self-regulation  of,  212 
Respiratory  and  circulatory  me- 
chanisms, co-ordination  be- 
tween, 212 

center,  210,  347 

cycle,  186 

gases,     interchange     of,     203 

interchange  at  high  altitudes, 

206 
at  low  altitudes,  208 

mechanism   of   mammals,    188 

movements,  effect    of,    on    air 
content     of     lungs,     200 
regulation  of,  211 
Respired    air,    changes    in,    201 
Retina,  378 

function  of,  388 

internal  limiting  membrane  of, 
388 

rods  and  cones,  387 


408 


INDEX 


Retina,  stimulation  of,  by  rays  of 
light,  387 

structure  of,  387 
Rigor  caloris  of  muscle,  85 

mortis,  chemistry  of,  90 
Rods  and   cones  of  retina,   387 
Rolando,  fissure  of,  327 
Roots  of  spinal  cord,  316 
Rotation  and  circumduction,  75 

SAC,  pericardial,  145 
Saccule  of  ear,  368 
Saliva,  226 

action  of,  231 

active  principle  of,  231 

character  of,  231 

glands  secreting,  228 
Salivary  center,  230 

digestion,  223 

glands,  228 

exherent  stimuli,  231 
inherent  stimuli,  231 
innervation  of,  229 
sympathetic,  230 
minute     structure     of,     228 
vasomotor  reactions  in,  178 
Saponification,  247 
Sarcolactic  acid,  89 
Sarcolemma,  62 
Scala  tympani,  368 

vestibuli,  368 
Sclera,  378 

Sebaceous  glands,  292,  293 
Secretagogues    as   stimulants   to 

gastric  secretion,  241 
Secretin,  237,  248 
Secretion,  216 

active  principle  of,  221 

movement  by,  49 

of  gastric  juices,  240 

of  pancreas,  220,  245 

of  urine,  288 
Secretions,  219 

classification  of,  219 

external,  220 

internal,  220,  295 
Segmental  animal,  307 
Self-regulation  of  respiration,  212 
Semicircular     canals,     368,     370 

ampulla  of,  371 
Semilunar  ganglion,  348 

pulmonary  valves,  139 
Sensations,  cutaneous,  352 

general  considerations,  352 


Sensations,   receptors  of,   classi- 
fication of,  354 
Sense,  dynamic,  345,  374 

muscle,  345 

of  equilibrium,  350,  362,  368 

of  hearing,  362 

of  sight,  375 

of  smell,  359 

of  taste,  357 

pain,  357 

static,  345 
Sense-organs,  352 

classification  of,  353 
skin  as,  354 

Sensible  perspiration,  293 
Sensory  epithelium  of  eye, 
376 

neurones,  95,  306 
Serum,  124 

Shortness  of  breath,  213 
Sickness,  mountain,  207 
Sight,  center  for,  337 

nerve  of,  347 

sense  of,  375 
Simple  brain,  321 

sugar,  231 

twitch  of  muscle,  76 
Skeletal  muscle  tissue,  57 
Skin  as  excretory  organ,  292 

as    organ    of    protection,    294 

as  sense-organ,  354 

layers  of,  292 

smooth  muscle  cells  of,  294 
Sleep,     effect     of,     on     arterial 

blood-pressure,  167 
Smell,  center  for,  338,  347 

nerve  of,  347 

sense  of,  359 
Smooth  muscle  cells,  57 

tissue,  59 
Sneezing,  189 
Sniffing,  361 
Soaps,  270 

Somatic  receptors,  354 
Sound  intensity,  362 

loudness,  362 

musical,  370 

of  heart,  145 

pitch,  362 

quality,  362 

waves,  362 

speed  of,  363 
Specific    gravity,  movement    by 

changes  in,  49 


INDEX 


409 


Speech,  center  for,  339 

loss  of,  339 
Sphincter  antri  pylori,  236 

cardiac,  of  stomach,  236 

of  pyloric  vestibule,  236 

of  urethra,  291 

pyloric,  of  stomach,  236 
Sphygmograph,  169 

Dudgeon,  169 
Sphygmomanometer,  163 
Sphygmotonometer,  163 
Spinal  cord,  312,  323 

as  organ  of  conduction,  314 

of  reflex  action,  316 
development  of,  311 
functions,  314 
roots  of,  316 

ganglion,  316 

reflex  animal,  311 
Spirometer,  200 
Splanchnic  nerves,  179 
Spleen,  302 
Spleen-pulp,  302 
Spot,  blind-,  388 

yellow,  390 
Spreading  of  impulses,  317 

of  reflexes,  317 
Stapes,  364 
Starch,  265 

action  of  ptyalin  on,  231 
Static  sense,  345 
Stationary  air,  201 
Statocyst,  371 
Steapsin,  247 
Stenosis,  147 
Stenson's  duct,  228 
Stentor,  57 
Stethophone,  145 
Stimulation,  intensity  of,  45 

of  light,  nature  of,  375 

of  retina  by  rays  of  light,  387 

phenomenon  of,  40 

strength  and  duration  of,  44 

threshold  value  of,  44 
Stimuli,  adaptation  state,  45 

classification  of,  42 

chemical,  44 

electrical,  44 

maximal,  44 

mechanical,  43 

minimal,  44 

optimum,  44 

photic,  44 

refractory  state,  45 


Stimuli,  subminimal,  44 
supramaximal,  44 
thermal,  44 

Stirrup-bone  of  ear,  364 

Stomach,  234 

bismuth  x-ray  study    of,    242 
cardiac  sphincter  of,  236 
contents,    evacuation    of,    243 

time  of,  243   . 
fundus,  235 

greater  curvature  of,  235 
lesser  curvature  of,  235 
movements  of,  242 
position  of,  234 
pyloric  sphincter  of,  236 
wall  of,  235 

Stored  fat,  271 

Striate  bodies  of  cerebrum,  328 

Striated    muscle    tissue,    57,    60 

Sublingual  gland,  228 

tonsils,  227 
j   Submaxillary  gland,  228 

Subminimal  stimuli,  44 

Sugar  in  urine,  290 
simple,  231 

Sulci  of  brain,  323 

Summation    of    muscle    contrac- 
tion, 80 

Superior     maxillary     branch     of 
trigeminus  nerve,  349 

Supplemental  air,  201 

Supramaximal  stimuli,  44 

Swallowing,  act  of,  233 

Sweat,  293 

glands,  292,  293 

Sylvius,  aqueduct  of,  310 
fissure  of  327 

Sympathetic  division  of  autono- 
matic  system,  351 

Synapse,  96 

Synergia,  343 

Synthesis,  262 

Systemic  circuit  of  circulatory  sys- 
tem, 151 

Systole,  136 


TACTILE  corpuscles,  354 
Taste,  center  for,  338 

sense  of,  357 
Taste-buds,  231,  357 

in  children,  358 

location,  359 
Teeth,  227 


410 


INDEX 


Temperature,  effect  of,  on  arte- 
rial blood-pressure,  167 

of  body,  278 

regulation  of,  279 
Tensor  tympani  muscle,  364 
Tentorium,  341 

cerebelli,  327 
Tetanic  contraction,  81 
Tetanus,  incomplete,  81 
Tetany,  298 
Thermal  stimuli,  44 
Thoracic  duct,  111 
Thorax,  192 
Threshold  value  of  stimulation, 

44 

Thrombin,  124 
Thrombocytes,  115 
Thrombogen,  125 
Thrombokinase,  125 
Thrombus,  126 
Thymus  gland,  302 
Thyroid  gland,  296 

extract,  298 
Tidal  air,  201 
Tissue  fat.  271 
Tongue,  226 
Tonicity  of  muscle,  69 
Tonsils,  faucial,  227 

pharyngeal,  228 

sublingual,  227 
Trachea,  183,  188,  190 
Translucent  bodies,  375 
Transmutation  of  matter,  32 
Transparent  bodies,  375 
Transverse  discs,  64 
Tricuspid  valve,  138 
Trigeminal  nerve,  348 

branches  of,  348 
Trochlear  nerve,  348 
Trypsin,  246 
Trypsinogen,  246 
Tubule,  uriniferous,  286 
"Tween-brain"  320 
Twitch,    simple,    of    muscle,    76 
Tympanic  cavity,  364 

membrane,  363 

UNDULATORY  theory  of  cause  of 
light,  375 

Unit     of     histological     measure- 
ment, 29 

Urea,  154,  270 
in  urine,  290 

Ureters,  285,  291 


Urethra,  291 

sphincters  of,  291 
Urinary  bladder,  291 

meatus,  291 
Urine,  acetone  in,  291 

albumin  in,  290 

amount  secreted,  289 

composition  of,  289 

indican  in,  290 

secretion  of,  288 

storage  of,  291 

sugar  in,  290 

urea  in,  290 

voiding  of,  291 
Uriniferous  tubule,  286 
Utricle  of  ear,  368 
Uvula,  226 

VAGUS  nerve,  350 
Valve,  aortic,  138 

mitral,  138 

tricuspid,  138 
Valves,    cardiac,    action   of,    139 

of     heart,     arrangement,     136 

pulmonary  semilunar,  139 

venous,  156    139, 
Valvular  lesions  of  heart,  146 
Vasoconstriction,  176 
Vasodilatation,  176 
Vasomotor  activity,  purpose  of, 
180 

center,  347 

mechanism,  175 

nerves,  175 

reactions,  different,  177 
in  heart,  179 
in  salivary  glands,  178 
to  heat  and  cold,  177 
Vater-Paccini,  corpuscle  of, 

355 
Vein,  hepatic,  250 

intralobular,  250 

portal,  249,  153 

pulmonary,  132 

renal,  285 

walls  of,  156 
Vena  cava,  128 
Venous  pulse,  170 

sinus,  130 

valves,  139,  156 
Ventilation,  205 
Venules,  129 

Vermis  of  cerebellum,  341 
Vertebral  canal,  312 


INDEX 


411 


Vestibule,  pyloric,  sphincter  of, 
236 

Vibratory  energy,  42 

Visceral  muscle  tissue,  58 
pleura,  192 
receptors,  354 

Vision,  direct,  390 
indirect,  390 

Visual  purple,  389 

Vital  activity,  262 

Vitalismus,  262 

Vitalistic  view,  30 

Vitamines  as  stimulants  to  gas- 
tric secretion,  241 

Vitreous  humour,  379 

Vocal  cords,  189 

Volition,  308 

Voluntary  muscle  tissue,  57 

WALL  of  alimentary  canal,   224 
Warm-blooded    animals,  279 


Warmth,    effect    of,    on    muscle 

contraction,  84 
Water,  absorption  of,  movement 

in  consequence  of,  48 
Wave  of  muscle  contraction,  71 

peristaltic,  60,  224 

sound,  362 

.speed  of,  363 
Wharton's  duct,  228 
White  matter,  cerebral,  arrange- 
ment and  structure  of,  324 
of  spinal  cord,  314 
Wintrich's  modification  of 

Hutchinson's  spirometer,  200 
Wirsung,  duct  of,  245,  252 
Writing,  center  for,  339 

X-RAY  bismuth  study  of  stomach, 
242 

YELLOW  spot,  390 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 


This  book  is  due*  on  the  fast  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


LD  21-50m-6,'59 

(A2845slO)476 


General  Library 

University  of  California 

Berkeley 


&10LOGT  UBRAR1 

10023 


